Another option would be to contact a local real estate expert in the area of where the home is located and ask for a Comparative Market Analysis (CMA) that could show you the sales history and current inventory. While a percentage increase is a good tool, it really is area specific as to what the current values are vs. when built and the level of demand.
Where is the home located? You should consider an appraiser who is experienced in the area. I would be happy to recommend a few with a location.
I agree with Lisa Lucky and Angela Zampino. Everything in a contract is negotiable between the parties. And, it is in a buyers best interest to contact the local branch manager of the real estate company of choice to find the most experienced agent(s) in the area.