I only do a few trackdays per year. Typically pay between£100 to £150 per track but that is more the later part of year prices.
Silverstone can be up to about £300 midyear!
I'll use about 1.5 tanks of fuel during the day. I clocked 155 miles on track at Oulton and worked out to be about 12mpg!
My car has never been standard but you could take a standard dc5 on track and be absolutely fine. The brake pads may start to fade but a good upgrade is Ferodo ds2500 pads (£100), braided lines (£50 ish) and dot 5.1 fluid (£20 ish).
Tyres are only good up to the limit of gri, so if your tyres aren't an expensive brand just don't take corners as quick. It's all about building up confidence first, then speed, and seeing where the car needs improving.
A fast road setup with lowered springs would be perfect - springs, front camber bolts, rear adjustable arms and geometry, maybe about £500 or so fitted and setup.
I don't service as such before and after, just ensure oil is topped up and relatively fresh. I would probably change before if it is say 8k miles old or so.
So a standard car will be fine to start with, all the Japanese motoring videos usually race a standard dc5.