If you OK with DIY, you can just get a solar panel fan, and attach it under the normal unpowered ventilator and pass the fan wire connecting to the solar panel through it.

MUCH cheaper, and more easy to replace. I did that on my roof which already had existing turbines, with an 8inch fan connected to a solar panel. You just have to secure the panel properly cause if strong wind it could fly off, I just put some extra roof tiles I had.