Tony,
In my honest opinion, your home depot 'greenhouse' enclosure does not provide anything for the monitor itself, aside from walls which prevent it from escaping.
The glass on the two side walls is a poor material choice for several reasons. Due to the nervous disposition of these animals, you typically want to restrict their visibility to any of your activities within the room. Visual blinds and visual barriers are important for limiting their stress levels and feelings of vulnerability- ESPECIALLY with this complex of monitors (which are wild caught).
Glass also provides very little in the ways of insulation (both keeping heat in, or keeping heat out). There are numerous other building materials out there which are much better used for enclosure walls (also keeping in mind that transparent walls aren't good for tree monitors). While temperature fluctuation may not be a big deal in your house, most people's homes experience some fluctuation in temperatures throughout the seasons. Proper building materials can limit this effect on the conditions within your enclosure.
Another problem with having the two side walls made of glass, is that you are taking away from valuable surface area which would otherwise be usable to your monitors if you made the walls climbable (by affixing cork, burlap, vinyl aquaculture netting, etc.).
There are many other reasons why this type of setup would not be sufficient or optimal for tree monitors, or other arboreal monitors, but I think that the few examples I have already mentioned would be reason enough not to use such an enclosure.
I strongly agree with what FR has already mentioned, a much better enclosure built according to a particular species' specific physiological and environmental needs (rather than buying something prefabricated and trying to 'make it fit' your animal's needs) can be built for substantially less than the price you quoted above for the 'greenhouse enclosure'.
Just my thoughts on the subject...