If it's the old-style Rockbass (distinguished by having a 1-piece bridge), then it's overpriced, and £200-250 is probably more like it.
If it's the new-style (with a 2-piece bridge), then it's probably on the high side but basically ok.
Having said that, they're fairly rare, so if you find the right buyer, you might get more money. But either way, I don't think it's going to be worth buying just to flog on.