How do convert decimal value to thousand

Here's a simple approach that will handle negative numbers, and give you the correct suffix for thousands, millions and billions:

public static string Convert(double value)
{
    double absValue = Math.Abs(value);
    if (absValue >= 1000000000)
    {
        return string.Format("${0:F2}b", value / 1000000000);
    }
    if (absValue >= 1000000)
    {
        return string.Format("${0:F2}m", value / 1000000);
    }
    if (absValue >= 1000)
    {
        return string.Format("${0:F1}k", value / 1000);
    }

    return string.Format("${0:F0}", value);
}

If you need different numbers of decimal places for the different suffixes, adjust the number after the F in the format string, or use your own custom format string.

If you want a different set of suffixes, add or remove if blocks. Just make sure you test the values in descending order. :)