If you got an Islabike you'd been a 700 large - but you could also go for a small adult cyclocross bike of any make. My son (10 nearly 11) has an Isla 700 small.
As for how much you'll pay - Islabikes have just had a big price hike and second hand prices are strong so you are probably looking at £250 upwards for an Isla700- you might get lucky and get one cheaper.
If you are looking new then Islas are £500 now I think and probably similar for the cheapest adult cyclocross bikes - I'd go second hand though as you don't want a bike they'll be worried about scratching and if you save money you can spend it on bits for the bike - maybe clipless pedals if they get into it - different tyres etc.
I wouldn't worry too much about the bike being a little bit big for them - kids grow quickly so if it fits bang on now it'll be too small in 6 months. Two of mine race cross and I coach our club kids so I see loads of kids on cross bikes and many race successfully on bikes that if they were adults you'd say were waaaaay too big.
Edited: 03/10/2012 at 23:36
