all depends how much you do yourself what kit you have and how good at sorceing parts you are.
helped a mate do his and we got that up and running for less than £500, £411 for a crap mk4 gti at auction and about £80 worth service items. its only a 180 brake lump but remaped it makes about 210 with map, but it'll do for now and it made it intresting to try and do it for sub £500 ironicaly the budget meant keeping the mk4 box. but it'll be getting a more intresting flavor of 1.8t when i get time to build it

at the other end of the scale 1.8 conversions get intresting again you can quite happaly blow 10k buy the time you've built an intresting engine, done some funky pipework and put a 6 speed box and slippy diff in there