I always leave my asking price in the ad. On AR15.com it is mandatory as it helps establish current values. Someone said the EE is not a Blue Book? I beg to differ, it is the most accurate gauge of current value we have. Just because someone asks for a certain price doesn't mean they'll get it, so while $800 for an SKS skews the numbers it will never sell for that price so that number is to be discarded.
As for asking price not being selling price, let's be real here. If you ask for $2,000 for a rifle that's worth $1,000 you ain't gonna get your asking price. Any number of multi-page ads with two dozen BTTs is evidence of this. Eventually your price will come down to a reasonable approximation of what you sell it for, and for measuring current values that's close enough.
If this were my site, keeping the price in the ad would be mandatory. You don't have to say what you sold it for or to whom, but your asking price was viewable the entire time the ad was up. Absolutely no reason to take it off.