i was recently checking prices for climbing ropes and they were quite a bit better than prices at other online retailers like backcountry.com
so maybe they do well on some stuff and not so much on others?
Traditionally, MEC's prices for climbing gear were the best going. Smaller shops would often match them, but you'd rarely find it any cheaper. I've been out of touch with climbing gear prices for about a decade, so I don't know if that's still true.
I assume it was simply their massive volume relative to what any other shop could move, but I've also heard that they subsidized core product lines (the co-op was founded specifically to get cheap climbing gear) with higher margin stuff like clothing.