Relatively cheap if you do it yourself......
Hire a circular saw, put step ladder next to Jasper, climb up, start cutting (don't worry about marking the hole out because you can guess it) Once the hole is there, finish off the sharp edge with some rubber edging, the roof itself can be made of tarpalin plastic (available from B&Q) the frame can be built (easily) from wood and simply nailed together. The whole ensemble can then be finished off with cable ties, magic tape, gaffer tape, duct tape and string.
For a neat and tidy job you'll need to follow my instructions step by step.
Otherwise it will look a right mess.