Formatting large numbers in C#

Slight refactoring:

public static string KMBMaker( double num )
{
    double numStr;
    string suffix;
    if( num < 1000d )
    {
        numStr = num;
        suffix = "";
    }
    else if( num < 1000000d )
    {
        numStr = num/1000d;
        suffix = "K";
    }
    else if( num < 1000000000d )
    {
        numStr = num/1000000d;
        suffix = "M";
    }
    else
    {
        numStr = num/1000000000d;
        suffix = "B";
    }
    return numStr.ToString() + suffix;
}

Use:

GoldDisplay.text = KMBMaker(gold);

Changes:

1. Explicitly use double constants
2. Remove duplication
3. Separate concerns - there's no need for this method to know about text boxes

i don't think there is a build-in method for that so i just maybe reinvented the wheel by writing my own way but this should be how i would maybe do it

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            for (double i = 500d; i < 5e23; i *= 100d)
            {
                Console.WriteLine(i.ToMyNumber());
            }

            Console.Read();
        }

    }

    public static class helper
    {
        private static readonly  List<string> myNum;

        static helper()
        {
            myNum = new List<string>();
            myNum.Add("");
            myNum.Add("kilo");
            myNum.Add("mill");
            myNum.Add("bill");
            myNum.Add("tril");
            myNum.Add("quad");
            myNum.Add("quin");
            myNum.Add("sext");
            // ....
        }

        public static string ToMyNumber(this double value)
        {
            string initValue = value.ToString();
            int num = 0;
            while (value >= 1000d)
            {
                num++;
                value /= 1000d;
            }

            return string.Format("{0} {1} ({2})", value, myNum[num], initValue);
        }
    }
}

which print this

500  (500)
50 kilo (50000)
5 mill (5000000)
500 mill (500000000)
50 bill (50000000000)
5 tril (5000000000000)
500 tril (500000000000000)
50 quad (5E+16)
5 quin (5E+18)
500 quin (5E+20)
50 sext (5E+22)