It seems like a reasonable price, but all they're selling is a basic laptop pc 40Gb hard disk in a small aluminium case. You could by a similar drive and the case seperately from many suppliers. The case usually comes with all the cables you need and instructions. It doesn't need an 'expert' to fit it together - if you can change a fuse in a plug, you should be able to do it quite easily.
If you buy a case that's just for USB (i.e it doesn't include a Firewire socket) then it will be even cheaper.
Of course, all this assumes you don't mind getting the parts and using a screwdriver. I reckon they're charging around £15-20 extra for having done that for you. Still, I don't think they are ripping you off