I haven't done it professionally in a long time, but e25 for D&P for 2 full sets of 36 5"x7"s is probably about the going rate.
Mad that it should be so much more expensive than colour, but 'market forces', etc... :rolleyes
Try ringing Leary's photo lab (Chelmsford Road, Ranelagh, don't have the no. to hand but they're in the book). They used to be fairly reasonable when I used them last, and they do good work.
Alternatively, do you know anyone in a camera club (maybe at school or college) with access to a B&W darkroom? Because the materials would cost well under a tenner...