FormatNumber.cs

using System.Globalization;

public static class FormatNumber
{
    public static string ToKMB(this decimal num)
    {
        if (num > 999999999 || num < -999999999)
        {
            return num.ToString("0,,,.###B", CultureInfo.InvariantCulture);
        }

        if (num > 999999 || num < -999999)
        {
            return num.ToString("0,,.##M", CultureInfo.InvariantCulture);
        }

        if (num > 999 || num < -999)
        {
            return num.ToString("0,.#K", CultureInfo.InvariantCulture);
        }

        return num.ToString(CultureInfo.InvariantCulture);
    }
}