Despite being a gardener since I was 9, 2012 has been my first season as a Greenhouse Virgin. It is incredibly exciting.
My greenhouse is aluminium (didn't have a choice as it was free) and is set onto a concrete surround. This helps to make it mouse proof. While the greenhouse was free itself, It has cost me around £850 to kit it out with timber staging, which was made by my husband, so there was only a timber and screw cost; 2 overhead lights, an internal tap for water; a waterproof electric plug for my hot box, proper flooring and sufficient weed suppressant membrane covered in chippings.
I have already had to ditch the automatic vent purchased as it wasn't strong enough for the job. My 4 roof vents cover 32 square feet.Unfortunately the next grade of automatics come in at £80 odd per vent..ouch.
All this cost however depends on what size you purchase and the rule of thumb is purchase a size bigger than you think you need. I went from zero to 10ft wide x 8ft high by 13 ft long (it had been cut down from 24ft long) and I still ran out of space.
One of the Dr Hessayon Expert series on Greenhouses will give you an excellent guide to being a glasshouse enthusiast. Good luck and happy growing.