While I wear bi-focals during my daily routine, I wear single lens, prescription Oakleys while riding. I do not require the bi-focal for riding, lucky me. They are awesome, well worth the investment. I struggled with other solutions for some time, before just saying "to heck with it" and taking the plunge. The wrap around style provides great protection, even from the wind.
I believe the old expression, "you get what you pay for" definitely applies here.