jonb is spot on. Well, dependant on how you paint it- I'm doing my old rigid with hammerite (er, have been doing it for about a year :roll:) and that's cheap- just strip and spray, 2 cans, £12 plus about a third of a can of nitromors and a load of time of course. But that's going to be a pretty rudimentary job, though it'll look fine and it feels lovely (smooth hammerite but with a dustcoat). even that makes no sense, I'm doing it pretty much for the fun of it.
What sort of paint are you thinking of? If halfords rattlecans or similiar, you'll need primer, your basecoat and a clear, and you'll possibly go through more than one can of the base and clearcoats. Plus wet and dry etc to cut back the clear, and a polishing compound to finish. That's a minumum of £25 or so. And add to that , that even if you get a great result (which you might) it won't last, the clearcoat is soft and WILL chip almost immediately.
BBQ paint is always a good option for DIY, tough and easy to spray and has great coverage. Looks like s**t but in a good way.