Custom Fields: Fee Formula

Hello. I just purchase DMS & Membership Pro and trying to create online journal subscription site.

One requirement is that we have a price tier based on the country of origin in public purchase. I am trying to do that with the fee formula in custom fields but not success. The fee does not calculate to the total.

Field Type: Radio
Values:
location A
location B
location C

Fee Field: Yes
Field Values:
10
20
30

fee formula: [FIELD_VALUE]+[dms_countryorigin]
dms_countryorigin is my custom field name

Anybody have experience in custom fields can help?? Thanks.