You can do it on the cheap for around £1,200, 1 x kit with bottle and all the relevant bits £600. But if you want a snazzy install and NOS computer controller them you're looking at the £1,200+ mark.
So if you went basic + Uprev and tuning you'll have change out of £2k. If you want stealth install, heaters, 2 bottles etc etc then expect about £4k.
It's a great in between stop gap or flirting with fun, but never a full time enjoyment factor. I had a 75 shot and enjoyed it, but the want for more power is hurtful on the wallet.
Personally get the Uprev and enjoy.