using System;
using System.Text.RegularExpressions;

public class Test
{
        public static void Main()
        {
            string response = "{lhs: \"1,0 U.S. dollar\",rhs: \"9 708,73786 Indonesian rupiahs\",error: \"\",icc: true}";

            Regex regex = new Regex(@"(?<=rhs: \"")(\d[\s,]?)+");
            Match match = regex.Match(response);
            string value = Regex.Replace(match.ToString(), @"\s", "");
            decimal decimalValue = System.Convert.ToDecimal(value);
            Console.WriteLine(decimalValue);
        }
}