A basic way of doing it would be: 
- Read the CSV File
- Sum the values
- Pass the ToStringvalue to SHA-1
public static string GetSha1Sum()
{
     double sum = 0;
     using (CSVReader cr = new CSVReader(myfilename))
     {
         foreach (string[] line in cr)
         {
             //assuming your field is the first: 
             sum += Convert.ToDouble(line[0]);
         }
     }
     SHA1 sha1Hash= SHA1.Create();
     byte[] data = sha1Hash.ComputeHash(Encoding.UTF8.GetBytes(sum.ToString()));
     string digest = HexDigest(data);
 }
The HexDigest helper is a function that format a byte array into hex string:
public static string HexDigest(byte[] data)
{
    StringBuilder sBuilder = new StringBuilder();
    for (int i = 0; i < data.Length; i++)
    {
        //format the bytes in hexadecimal
        sBuilder.Append(data[i].ToString("x2"));
    }
    return sBuilder.ToString();
}
More information about the CSVReader