As you know, fees are often displayed in different ways (i.e. $$$, 3$, 3, 300, $x3). Some reviewers even attempt to be more creative
I would suggest that you consider creating a dropdown menu from which the fee may be selected. Sure, choices would have to be limited somewhat AND someone will pay a fee that may not be specifically listed in the menu, but it would be easier to follow...in general.
It is also possible to limit the range/type of characters that may be entered in a field. For example, 130 is okay, but 1.3 is not an authorized entry. If you hobby at all, you are familiar with unauthorized entries...
Just my $0.02, .02, 0.01x2, 2(100/10,000)...you get the point