You'd be looking at around £100 to have frame enamelled one colour. More if it's a flam or metalllic colour.
If you want to do it on the cheap, go for powder coating instead. It's cheaper and tougher, and can look very nearly as good. That'll run to roughly £50 for a frame.
If you're tempted to spray it yourself, go for it, but be warned that it's not easy to do right, and the finish won't be as nice or last as long as stoved enamel or powder coat, and rattlecan enamel could end up costing you as much as a pro powder coat anyway.