Given a typical movie ticket price of $10, I think $20 is just about the drop-dead barrier for first run, single-viewing rentals.
I have a 1080p projector and 106" screen, and often delay seeing movies to wait for the Blu-Ray, which I can often get for less than $20. I view it as spending $10 or less per "ticket" (I imagine most people view a movie with one other person). But it is true of course that repeat viewing is possible. I can't imagine going much above that for a single viewing with an average of 2 viewers.
I think the notion that home viewers would be cramming their houses with friends and relatives to take advantage of a single-viewing rental is ridiculous.