You can upgrade to a Vortex. They have a single speed and a 3 speed. The best part is they are inexpensive and you get to keep your existing vent/vent cover/lid and all that. As this is replacing an existing fan the wiring is already run. No special tools needed.
Most other fans you rip out the entire vent and replace all of it, spending some time on the roof. Not with this solution.
They do tend to be a bit noisy, but not too bad for a bathroom.
I wrote this about putting one in my living space.
Vortex Vent Fan Upgrade: Install and Review
Under $100. Camping World has them, other places too, sometimes cheaper.