Here you go. Prices starting from 99p.
http://www.ebay.co.uk/sch/i.html?_trksid=p2053587.m570.l1311.R1.TR10.TRC0.A0.Xmountain+bike+wheel&_nkw=mountain+bike+wheels&_sacat=0&_from=R40
DO NOT put a £20 wheel on her bike! You get what you pay for. I once forked out £150 on a pair of racing wheels and they were only entry level. £20 wheels are dangerous and likely to break very easily. Even the ones you think are costly are likely to be the lowest quality. If you want to spend...