As far as the LFS, I think it's just easier to sell frags for $10, $15, $20, etc. this way the math is easy and they can do deals like $10 each or (3) for $20. The size if the frag can vary greatly, but the more inexpensive corals are decent sized for cheap.
Once you get into chalices, SPS, and some high end LPS and zoos, a frag can be over $100, or in the case of zoo's, depending on the colors can be up to $100 per polyp that's 3mm across!