@Natalie Kolodij
I'm not arguing with you at all. Maybe I misread the situation. I thought you mentioned previously the house was worth $265K based on the comps you were seeing (Owner said $315K based on comps as well). Now you're saying $260K. OK. I also got the impression you did like the house and wanted to stay. Your very nice landlord thinks it's worth $315K. Is he totally nuts? I don't know.
I do know that on any deal there is a fair range where there is an equilibrium between buyer and seller dictated by the current market. Without looking at a single comp (I could be WAY off) I would guess that range for this home in it's current condition to be $275K - 295K.
You and your landlord seem to be reasonably intelligent people with a case to be made. Compromise is how you buy this home and he sells it. If you don't want to buy this home at a price he's willing to accept you should let him know so he and you can move on. See, the problem is, he can list the home in a seller's market. If your price is to low, that's exactly what he will do.
Just trying to help! Sorry if I'm assuming too much but doing my best with the info I have available!