There is about 2" extra room in the stretch seat exit rows compared to the first three rows of the plane, though I have never seen Frontier charge a different amount for the exit row than the front rows.

The exit row stretch seats typically sold before the front rows, so perhaps this is just another way to try and find a few extra $$ where they can.

In my opinion, it is not worth the extra $13 for the exit row compared to a regular stretch seat up front. I'd go for row 2 or 3 if available (bulkhead row 1 has less space for your feet).