Being in the trade i pay about £40 per litre for basecoat paint. To respray a medium size car ( Escort for example) i would use about 2 and a half litres. (and also 2 and a half litres of thinners)
The basecoat then needs coating with clear lacquer. I would use approx 2 litres of lacquer and this is mixed with one litre of 2k hardener.
Cost of lacquer and hardener would be approx £35.
These are trade prices and retail prices could be about double!
You may well be able to buy the paint and lacquer cheaper as there are a number of ‘cheap and nasty’ brands on the market but the prices i’ve quoted is for good quality materials.