im probally not the best person to ask as ive not acctually hit the tracks yet, Im just working off the advice of others. however so far its not been too bad, The mods ive baught Especially for the track so far have been front drilled disks and pads which cost a little under the £100 mark from GSF, The seats which are usally £130 each + £120 for two subframes were a total bargin. £50 a seat and £30 for BlueMk2 and his dad to weld me up some subframes. Thats more or less it so far, The price goes up depending on how good you want your parts.
next step should be Coilovers (between £300-£500) and a better Exhaust system (£300), however the exhaust system is not Vital, Ive got my mind on a Roll cage but they seem to go between the £500 - £1000 depending if you get a full or half cage.
Persionally ive spent £300 just on track day parts, Will probally end up spending around the £1000 mark including harnesses, cage etc. The cheapest mod so far has just been gutting the interiour, Its also given me the best results, Cost me a fiver for a crowbar and a set of screw drivers to do it.