I'd pay no more than about Â£5, they're pretty common.
BTW these adapters don't allow 60Hz output, they simply trick the game into detecting a 60Hz console (when the system is really at 50Hz). The only way to get 60Hz output from a PAL MD is via an internal mod (i.e. fitting a 50/60Hz switch).
Maybe you knew that anyway, but I thought I'd better mention it.