The vote that you have started here is very interesting to me as a portal manager. It seems as though it doesn't matter how much it costs to list on a portal, the rental owner wants a multitude of enquiries. I can understand the thinking behind this, as most owners do not make a profit on their renting, so any spending that they do on advertising has to make a return, and a good one at that.
In my estimation, an owner would need at least 3 rentals for them to bother re-advertising.
I hope this vote gets a lot of interest.