Are you sure a roof fan is your only/the best option?
In many cases, putting in soffit vents and a ridge vent for passive air circulation is more than enough and then you don't have the electricity charges later. The hot air rises and goes out the ridge and is replaced by cool air drawn into the
soffits. The beauty of
a ridge vent is that even though you are cutting a long slot at the peak of the roof, there is very little water to deal with at the peak. So you just nail or screw the ridge vent down. You don't need to worry about flashing around the opening.
Some are even low profile so you don't notice them.
If you check out building science's
web site, they have a lot of detailed information on the best way to ventilate an attic. They believe in soffit vents and ridge vents. I don't know where you live, but they have lots of
different models for the best way to build houses in various locations.
Here's an example for a house in Charlotte, N.C.
Another option is to put
gable vents in the attic walls at the ends of the house. You don't really have to worry about water there either. You can put one in each end, and get cross breezes, or you can put
a fan in as well.
I don't know your house layout, or your location, but for my house, a
powered roof fan would be the last option. They are ugly, consume electricity, and you have to worry about flashing around them so they don't leak.
Don't forget that you need to put a vent somewhere else to let air into your attic to replace any air you try to blow out. If you don't, you can suck air from inside your home, and this is often replaced by air back drafting down your chimney. Carbon monoxide is a concern then.