There is a thread somewhere on the forum where Frank and Jefferson talked about the max price of a home to fill a vacant lot given a certain lot rent, but I can't find the thread anymore.
If I remember correctly, the discussion went something like this...
If your lot rent is Y, then the max price that you would want to pay for a home to fill the lot = Y x 7 x 12. For example, if your lot rent is $200, then the max price = 200 x 12 x 7 = $16 800.
I think Jefferson recommended Y x 8.5 x 12, e.g., 200 x 12 x 8.5 = $20 400.