$/per square foot is a short hand IF everything else is the same. Same age, condition, quality, location, schools, etc. When you through in land, unless you have other similar comparable sales, I wouldn't trust this calculation too heavily. It's A factor, not THE factor.
A great local agent can provide you with some other comparables and do some work with you to come up with a fair price to offer.
Getting an offer price online or from people who haven't toured the home isn't reliable to make any decisions off of.