Assuming that I've started knowing the focal range that I want, I look at several things. I usually look at MTF charts first. I read professional reviews. I talked to people that I know and whose opinion I respect.
I check user reviews, but when I do so, I'm not very concerned with their assessment of image quality. I'm usually on the lookout for complaints. I usually skip over the positive user reviews.
My biggest selection criteria (after suitability) is usually quality rather than price. For my core lenses, I use them so much that the cost per use becomes almost trivial. On non-core lenses like fisheyes, macros, or other things that I won't use very often, I flip that and focus heavily on price rather than quality.
I would also be much more likely to focus on price rather than quality if I were buying a lens that intended to replace in the relatively near future. For example, if I were buying a mid-range zoom knowing that I wanted the 24-105 f/4.0 IS or the 24-70 f/2.8 bought couldn't afford them for a year or two, I'd be likely to buy something cheaper than usual to get by with. I suppose resale would come into play in that decision, but I rarely ever sell anything so I usually ignore resale value.