what i found on both my builds ( not on the one i did for dad though hes a real tight arse bast@rd) is that it doesnt get expensive until the end. It depends on the quality of finish you want, a nice set of dials can cost 500 quid and then theres things like fuel caps, lights, switches, nice seats etc. IVA and registration/tax/insurance is about a grand. It also depends on what time of timescale you set yourself. Its always going to cost you more if you build your car within say a year than if you build it over say, three years as you can pick your bargains at the right time. K4Kevs build is a prime example, hes been hording parts for years
(sorry kev!)
Best way of doing in my opinion it is to get it on the road as a basic car with basic fittings, and then upgrade it as you own it, at least that way you can have some fun driving it