I wear basic clear safety glasses over my contact lenses. They give a lot more protection,especially wind resistance,than my prescription glasses. I wear a visor on my helmet to keep the rain drops off the specs,but no matter what you do they always get some vision obstructing rain drops on 'em. Carry some kitchen towel to wipe the drops off while you're waiting at the traffic lights etc.
I do exactly the same, I'm -4.5 prescription so cycling without some kind of eyewear is not feasible. I wear daily disposables for cycling only, and just throw them away after a ride. They're really cheap nowadays, and I don't cycle every day so they end up even cheaper as a months supply lasts a good while.
As for glasses, I use Bolle Contour safety glasses which are about 7 quid a pair on
ebay. I've got both clear and smoke tinted versions and to be honest, there's no difference between them and cycling glasses.
I bet if you put these on sale in a bike shop at £15 or £20 a pair people would snap them up.
If it chucks it down I can put up with the rain drops on the lenses, take the glasses off for a while, or just push them down my nose a bit and look over the top of them.
I can't do that with normal glasses as it's all a blur!