Commons aren't very expensive though. The biggest ones I've seen, some over 14" aren't more than $30 but I guess it depends on the area and the store. $40 just sounds outrageous to me.
A lot of times you can get large commons for free, as people are not always aware of their size when they buy them. I got mine who is over 16 inches for free from a person who was more than happy for me to take it off her hands.
What I do when I am looking for a large fish such as that is I go to a lfs and give them my name and phone number so that they can get in touch with me if someone wants to dump a large fish on them. Many times a person will call the store and say "Will you take my large pleco" and then they can just give my or your information