I did it for about 2 years was a lot of fun and about 3 times as expensive as rc car racing, a decent bike 2nd hand is gonna be £1000 + you need reasonable helmet gloves minimoto knee sliders.
Good fun but I get mutch more enjoyment from my gsxr 600 track bike which only cost me £800.
bear in mind a set of real motobike tyres costs £200 a pair of mini moto racing tyres is about £100.
I used to race here.
http://www.minimoto.co.uk/
loads of info etc on the site, if you do get one get a 4.2 hp air cooled as the higher hp bikes require mutch more in parts etc.
believe it or not I have some trophies from my mini moto racin days
http://farm4.static.flickr.com/3022/...838906a9_b.jpg