At minimum wage £100 is around 19 hours work, I earnt more than that for 1, 8 hour day teaching, I don't think its a rip off for the work involved but will concede its a lot of money when the original plan was to DIY. I have probably spent twice that amount of time so far to dig over around 1/3 of my plot (about 60'x12 growing space), but am going slowly and being very thorough, no guarantee others would be as thorough as we are ourselves. I would go the route of finding friendly neighbours/teenagers/family etc before forking out to pay someone though, and whoever did end up doing it I'd be there supervising to make sure it was done properly!.