Chris, a roof top garden sounds like a wonderful place to observe from! You could probably start your project small and with minimal expense by mounting a metal pier to the roof. A length of 6" iron pipe with the proper flanges welded top and bottom to bolt it to the roof and and to hold the CG5 without the tripod shouldn't be too dear in cost, perhaps 100 pounds. Properly painted with rust proof enamel and filled with some sand it should be steady enough for visual observing. Once aligned you could just slip the C8 out of the dovetail mount and cover the pier/CG5 with a tarp and take just the telescope in for the night. In fair weather even the telescope would be alright outside under a cover occassionaly.

A proper roll off roof observatory costs depend on its size and the cost of timbers/hardware and the labour to construct it if you're going to let that job out to someone. Many people build them as a personal project and do the labour themselves saving on that expense. While not difficult to do if you have the time and tools to construct it, being at university may mean that your time is limited and your budget to purchase tools may be limited as well. At a minimum you'd need a proper circular saw, a sturdy drill, spanners/sockets, a tape measure, a hammer and all the other bits of kit for building a shed. You can save on the cost of a saw and the mess one makes by having the shop where you buy your timber cut everything for you but your measurements need to be dead on and what they charge to cut it may be more than the cost of the circular saw in the end. You can purchase a prefab metal or wood shed and just modify the roof so that it moves or tilts, lots of people do this and are very pleased with the result, it may actually be the least expensive and quickest way to get a structure built. So long as the unassembled bits will fit in the lift it may be the way to go.

Take a look through the "post a picture of your observatory" thread and you'll see lots of observatories from the magificent to the humble. If you see one that you like then PM the builder/owner and strike up a conversation. We all love to talk about our observatories if givena chance.

Again welcome and good luck!