To answer your original question, its never worth buying cheap.
When i first started trials on an old tbird, i used a bmx freewheel, and snapped them every 2 weeks.
A freewheel which slips will always make u less confident with your bike.
Ive never had my echo sl slip (and i bet everyone who's had one, will agree with me).
i wouldnt buy any other freewheel Woth every penny of the £60