I have never used a greenhouse exclusively for reptiles but I do run a teaching greenhouse for poinsettias/bedding plants. Occasionally I will take a Phelsuma cage in for some sun. I wouldn't recommend building a greenhouse just for reptiles unless you have some kind of horticulture need as well. To really make that structure suitable for your herps you will need to heat and cool it. Greenhouses hold almost NO heat. Especially the kits, they usually have single, thin coverings. Greenhouses also collect a lot of sun and really heat up in the summer, just like your parked car with the sun shining on it.
I'd imagine that it wouldn't be economically feasible, but a cool idea.
-Jake