using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var test = "ABC $300 DEF $600 GHI $120 JKF $980  lkaj $42,134";

        var rx = new Regex("[$]([0-9]{1,2},)?[0-9]{3}");

        var matches = rx.Matches(test);

        foreach (var match in matches)

        {

            Console.WriteLine(match);

        }   

    }

}