I never found price per sq ft. to be helpful for residential units since so much more goes into the quality of the space than just pure area. It's not like office/warehouse space where square footage dictates usability.
Do as others have already recommended. Look at the price on an absolute basis and see what the comps in the area look like. Offer to split the difference if you think it would be a fair price.
Also, sometimes upgrades are worth paying for upfront, especially if they've done major items like window replacements, new HVAC, or a bathroom/kitchen remodel. $10,000 spent now could be $15,000 you don't have to pay for out of pocket later.