Not sure, maybe £300 to £500? If you were to add up the cost of the materials:
10L of dip for about £140
Spray gun (you get what you pay for! Or just use a mates..)
Realistically it's pretty cheap - probably about £160-£200 once you've bought other bits and pieces and given the car a detailed clean, just add on the labour costs if you chose to get someone else to do it. A clean environment is preffered, but you can do it on your driveway if you like.
Me? I'd prefer a vinyl wrap! Could even wrap it myself which is a plus ;D