I think the lower price for CAT5 is based on demand/sales volume. Having said that, I would be shocked if any reputable label sold 25 pair CAT5 cable for less than 25 pair CAT3 cable.
If the price is reasonably close, based on the small amount of cabling I do, I have no worries about going with CAT3 if that is the appropriate cable. I just wish Commscope shipped cable that was correct from the ground up. Now I'll have to untwist the pairs to punch them down on 66 blocks...but...termination on the keystone jack side is more like CAT5 and CAT6.
