Vague.
I'm assuming you want to either;
A) Repaint the bonnet of the car a different colour to the rest
B) Have got into a little scrape and need to repaint the section that is scratched.
Answers follow...
A) Don't do it, looks awful.
B) If you can afford to get the whole car done then get it done, having a single panel that has been repainted even in the exact same colour looks really different to the rest of the car. The paint ages with the car and you can plainly tell the difference in colour.
Prices vary massively as far as I know. You could be charged anywhere from £750-£1000 for cheaper places to upwards of £2000 for the more expensive paints and paintjobs. You're best off phoning places up and asking though as i've been quoted much more than 2000 quid in the past.