Well I just read the RPData report and the lowest price was 340k BUT there were some for $0. I'm assuming that these may be recent sales for which the figures aren't yet available.
I appreciate that I need to be going with the "market value" but as much as I do like it, it doesn't feel worth more than about 320k to me in terms of rental return in the future and also holding costs.
RPData reports are ok for providing an overall picture but by knowing the market, I mean getting out there by foot and physically inspecting and knowing why one property sold for more/less than another. When a sale is $0 or very low it's usually an unreported agent sale or a family transfer.
As for "feeling that it's not worth more" you are letting emotion cloud your decision, not the numbers. However if $320K is your cutoff for your purposes and cash-flow position then you have your answer as to your maximum price. Best of luck with the negotiations