I've spent years buying expensive glasses only to realize late in the game that they're expensive mostly because only a few companies control the market in the US (check out http://www.cbsnews.com/8301-18560_162-5 ... expensive/
for information) and that you can get good quality glasses for much less.
Some websites we've used have been zennioptical.com, coastal.com, and warbyparker.com. The last is slightly more expensive than the others but will ship you five frames of your choice at no cost for a home try-on; they also have outstanding customer service. I've also heard good things about Costco for glasses but have never used them.
One thing to pay attention to, whether you buy online or in a store, is the quality of lenses that you purchase. If you have a high prescription this matters more, but in general the cheaper pairs of glasses tend to use a polycarbonate lens, which I've heard isn't as good. (I have not been able to tell the difference -- but maybe this is because I either have too little a vision impairment for it to matter or am just too blind to notice
There are some good threads on Bogleheads about this.