They come up for sale very rarely indeed in the UK. Haven't flown one but everyone I know who has likes them a lot. They do appear to fetch serious money on the few occasions when they do come up, like anything from £50K upwards assuming the engine has a bit of life in it.
You may do better trying to buy into a share of one that's recently been completed. A builder can pretty quickly bring back his cash outlay on the kit by syndicating a share or two out. Unlike most other kitplanes, RV builders quite often seem to be serial offenders - the Vans newsletter has a lot of stories of folk who finish one, and start another more or less straight away. Some people have built three! e.g. His and Hers combinations and "This one's for the kids to learn to fly in".