To be honest, I wouldn't use a Regex - I'd use TryParse:
NumberStyles styles = NumberStyles.Integer | NmberStyles.AllowCurrencySymbol;
CultureInfo provider = new CultureInfo("en-US");
if (int.TryParse(stringToConvert, styles, provider, out number))
{
if (number >= 2000 && number <= 1000000)
{
...