Check out the Vortex fan by Heng's Industries. It's a great high CFM fan that fits into your existing 14" vent frame. I used one on my JayFlight and it works great. Installs in about 30 min without breaking the factory seal and can all be from inside.
http://www.amazon.com/Hengs-90043-CR.../dp/B002N1J5IM
Check around, it can be found for less than $50
You may want a vent cover to run I in the rain. I used MaxxAir II, again I'm very pleased and it installed easily without effecting the roof seal