I think that all you need is to enlarge that field, so to not see the "?" anymore.
Perhaps your field needs to be resized to be a bit wider.
You can get the ? if the field is too narrow to display the value and you can get it when the value is invalid such as a divide by zero calculation or an invalid date, but since this doesn't appear to be the case here, I'd check the width of the field--espically since the currency formatting will require more width in order to be able to display the currency symbol.
You can check that possibility by just clicking/tabbing into the field while in browse mode. If the only issue is that the field is too narrow, it will pop up and display the value correctly when you enter the field. If there's another cause for the question mark, it'll stay a question mark.
Arggggh. Resizing the field did it!! Thanks to both posters.