"fair" is whatever the buyer and seller agree to and find mutally beneficial. As every situation is different , as are every buyer and seller there are no "norms" on these transactions. Might I suggest however, that what ever the two sides (buyer/seller) come up with is handled with an attorney. Yes, even if you are related !
An agreed purchase price is agreed to at the onset, not after the buyer's wear and tear on the property. Therefore, the buyer does NOT benefit from financially from his own wear and tear. You may want to consider some protection for yourself, that if in fact they eventually back out of this agreement that excessive wear and tear would be financially addressed, once again, in writing through an attorney.
I hope this works well for you, Let us know