You don't install solar attic fans to save money. You install them to supplement the existing ventillation in your attic in an attempt to reduce the super heated temperatures that can occur in the summer.
I had two solar attic fans installed by The Solar Guys. One near the front of the house near the garage and the other towards the back near the Lanai. Solar fans are now so efficient that they come very close to an A/C powered fan. The benefit is you don't need to run electrical wires to the fan.
In my case I have a WiFi thermometer in my garage attic. I only have ridge vents. Before the fans the temperature in the attic would get 125 degrees or hotter on the hottest days. After the fans I've not see the temperature rise more than 10 degrees hotter than the outside air.
The benefit is stuff stored in the attic doesn't melt any more, maybe my A/C has a little less load so I could be saving a little money, my roof may last a little longer and I don't have to worry about moisture or superheating issues anymore. My main benefit is my attic remains at a reasonable temperature and doesn't super heat.
One word of caution. There are two sizes of solar fans that the Solar Guys sell. In my case I have the more powerful one in the front of the house so it ventillates the garage attic better and the less powerfull one in the rear. Initially I had two of the large fans installed and on a quiet day in the afternoon when the sun is beating down on the fans you can slightly hear the large fan over the living room. The point is try not to get the largest fans if you don't need them. I have a 2600 sqft Ivy.
I also agree more insulation is good to but I really feel you need both.
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