Yes, it's actually a very rational behavior. Personally I haven't paid more than $10 for a game in... a long time. Don't remember exactly how long. And generally I don't pay more than $5.
But that's largely connected with the fact that I have very little time to actually play games (so many of the games I get on steam I never actually play more than about 15 minutes), and have more fun making them anyway.
We don't expect any remotely high percentage of our sales to be full-price sales. 85%-off is quite a bit lower than we like to go but if it broadens our customer base and the number of folks getting their friends interested, etc, then it works out.