Having helped many 100's of newbies to start up, it would be most unusual to spend less than £1000 even on a DIY system. By the time you have bought a decent sized tank, a good pole, a decent RO Unit and all the fitting kits, straps, wiring, switches and bits and bobs, you would have very little change out of £1000.
Several firms do supply DIY kits that are good value, however, these are usually just a starting point and will need several upgrades and plenty of other bits and bobs to get you actually working.