I think it's a good idea, but yeah it's expensive. As to greenhouse, not any more so than any other kind of a lid. Yes, if your tank water gets too hot in the summer, as in you have no A/C, blowing a fan across the surface is a cooling strategy. I had one summer without A/C where the house got to 98 during the day and the water temp never got over 79 with a 30 gallon tank. I never needed to do the fan thing.
Do you have a heat prob with your tank? If not, this should not be an issue just because of the lid/hood. The glass will transmit the ambient temp quit well. That's how those stick on liquid crystal thermometers work and also why they don't work on acrylic tanks (at least they didn't for me). But even if you have an acrylic tank, it is your ambient room temp and heater are most influential on your tank water temp. If you have very strong hot lights they can be a factor, but not the lid in and of itself.