From a busniess point of view, you need to move him out. Right now he's taking up a whole tank, and thats costing you money. If he was gone and you stocked the tank with new fish, and sell 10 of them in the next month, and make $5 on each one, thats $50 of profit.
If that big lard-ass pleco is still sitting in there with a high price tag you are out of pocket, and would have been better to just give him away free with the purchase of a jar of algae tabs.
By all means put $70 on him, if you think thats what the local market will bear, but if you dont get any interest, or someone likes him but baulks at the price, cut the price and move him out.
Ian
P.S. I seem to get given plecos all the time as friends and family know I have big tanks, so I have one just like that. As I can get them free, or buy a small one for $10 and wait, I'm not going to pay $70 for one. But I also understand that as a business a shop has to make something when they re-home big fish like that.