Well my "common" plecos were around 14" when I sold mine and I got $35 a piece for them. The price may be a bit high, but for people with larger fish that require new tank mates to be big enough in size to not be eaten/seen as food, there isn't always large sized plecos available so people can't always find suitable sized plecos to get. So since suitable size plecos aren't always available, some are willing to pay a higher price since it's the only suitable sized plec around.
But prices in your area might be different.
It does seem messed up to "profit" off my plecos, but I really wasn't trying to profit, just trying to recuperate the costs of their algae wafers. Boy those big plecs sure eat a lot of algae wafers, and I was feeding the expensive Omega One Algae Rounds wafers.
Selling doesn't really mean you will find a "better" home, you actually might drive away great homes because the price is high. Priced cheap or free could have any randoms inquire about the fish, but verify that they have the proper needs for the fish (ask for pics, tank mates, tank size, etc) just to make sure that you aren't rehoming the pleco to a miserable home where it might die.
I even had some "buyers" that I didn't feel were so great of homes, so I passed on them. I put the fishes' well being first. If there was a great home that wanted them for free, I would give the fish to them.