Thanks! Conversion rates for campaigns depend on the type of site, traffic quality, the targetting options you choose and ofcourse - banners and offers. But in general - they are lower for campaigns targeted to visitors that don't engage and want to leave straight away (0.1-2%), and higher when users have spent some time on the site, and really high (10+%), when campaigns are shown to visitors who started purchase process but for some reason decided to leave.
This would suggest that the only ethical use of this is for encouraging conversions of those that really do want to go ahead but are wavering because of the cost. What we need is some sort of extension that will automatically detect if a lower price is being offered if you back out :)