The easy way is to put the data into a flat object like a datatable and then sort :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;

namespace ConsoleApplication98
{
    class Program
    {
        static void Main(string[] args)
        {
            Response response = new Response() {
                Makes = new List<Make>() {
                    new Make() { 
                        Name = "AAA", Agents = new List<Agent>() { 
                            new Agent() { 
                                Id = 123, Name = "SSS", Offers = new List<Offer>() { 
                                    new Offer() {  
                                        Id = 100, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}},
                                        },
                                    },
                                    new Offer() {  
                                        Id = 200, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}}
                                        }
                                    }
                                }
                            },
                            new Agent() { 
                                Id = 456, Name = "SST", Offers = new List<Offer>() { 
                                    new Offer() {  
                                        Id = 100, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}},
                                        },
                                    },
                                    new Offer() {  
                                        Id = 200, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}}
                                        }
                                    }
                                }
                            }

                        }
                    },
                    new Make() { 
                        Name = "AAB", Agents = new List<Agent>() { 
                            new Agent() { 
                                Id = 123, Name = "SSS", Offers = new List<Offer>() { 
                                    new Offer() {  
                                        Id = 100, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}},
                                        },
                                    },
                                    new Offer() {  
                                        Id = 200, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}}
                                        }
                                    }
                                }
                            },
                            new Agent() { 
                                Id = 456, Name = "SST", Offers = new List<Offer>() { 
                                    new Offer() {  
                                        Id = 100, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}},
                                        },
                                    },
                                    new Offer() {  
                                        Id = 200, Models = new List<Model>() { 
                                            new Model() { Name = "TTT", Price = new Price() { Total = 1.0M, Vat = 2.0M}},
                                            new Model() { Name = "UUU", Price = new Price() { Total = 2.0M, Vat = 3.0M}}
                                        }
                                    }
                                }
                            }
                        }
                    }
               }
            };

            DataTable dt = new DataTable();
            dt.Columns.Add("Make Name", typeof(string));
            dt.Columns.Add("Agent ID", typeof(int));
            dt.Columns.Add("Agent Name", typeof(string));
            dt.Columns.Add("Offer ID", typeof(int));
            dt.Columns.Add("Model Name", typeof(string));
            dt.Columns.Add("Total", typeof(decimal));
            dt.Columns.Add("Vat", typeof(decimal));

            foreach (Make make in response.Makes)
            {
                string makeName = make.Name;
                foreach (Agent agent in make.Agents)
                {
                    int agentID = agent.Id;
                    string agentName = agent.Name;
                    foreach (Offer offer in agent.Offers)
                    {
                        int offerID = offer.Id;
                        foreach (Model model in offer.Models)
                        {
                            string modelName = model.Name;
                            decimal vat = model.Price.Vat;
                            decimal total = model.Price.Total;

                            dt.Rows.Add(new object[] {
                                makeName,
                                agentID,
                                agentName,
                                offerID,
                                modelName,
                                vat,
                                total
                            });
                        }
                    }

                }
            }
            dt = dt.AsEnumerable().OrderBy(x => x.Field<decimal>("Total")).CopyToDataTable();

        }
    }
    public class Response
    {
        public List<Make> Makes { get; set; }
        public Response()
        {
            this.Makes = new List<Make>();
        }
    }

    public class Make
    {
        public string Name { get; set; }
        public List<Agent> Agents { get; set; }

        public Make()
        {
            this.Agents = new List<Agent>();
        }
    }

    public class Agent
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public List<Offer> Offers { get; set; }

        public Agent()
        {
            this.Offers = new List<Offer>();
        }
    }

    public class Offer
    {
        public int Id { get; set; }
        public List<Model> Models { get; set; }

        public Offer()
        {
            this.Models = new List<Model>();
        }
    }

    public class Model
    {
        public string Name { get; set; }
        public Price Price { get; set; }

        public Model()
        {
            this.Price = new Price();
        }
    }

    public class Price
    {
        public decimal Total { get; set; }
        public decimal Vat { get; set; }
    }

}