Try:
string amount = DropDownList1.SelectedItem.Text;
Regex regex = new Regex("(\\d+(\\.\\d{2})?)(?=\\))", RegexOptions.Multiline | RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
Or
string amount = DropDownList1.SelectedItem.Text;
Regex regex = new Regex("(\\d+(\\.\\d{1,2})?)(?=\\))", RegexOptions.Multiline | RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
Which will also allow (99.9)