I've had glasses since i was 8 and prisms since my early 20s (since i moved here actually, Dr Bizers in Louisville never put them in for some odd reason!), and i find they usually charge the earth for prisms :/
Tescos say they won't, even with prisms in, the lenses are free - all you pay for is the frames.
Tescos say they won't, even with prisms in, the lenses are free - all you pay for is the frames.