Lenses for shooting glasses, either from the manufacturers or "specialist" outlets are at pretty imaginative prices, to say the least.
I've just had a couple of different strength prescription lenses made by:-
http://www.reglazemyglasses.com/
Their normal price for a pair of lenses is £20 (same as for a pair of single vision spectacles), but at least once a year, they have a £10 off promotion, which gets flagged up on:-
http://www.moneysavingexpert.com/
I've now had 3 pairs of specs plus the shooting lenses done by them and am 100% satisfied. If you have any queries, ring and ask to speak to Jamie (the owner). Very knowledgeable and patient.
At their prices, you can afford a set of different lenses for different lighting conditions.