Some slight ramdom paper reading, back in my uni days. Though I've ran across it via other sources over the years since. Unfortunately I don't have any links to hand though.

It might better be described as people put numbers into categories. Most people have a 10-20 category. 19.99 fits. 20.00 gets bumped up to the next box. It's a sub/semi conscious thing. If we use our higher thought process, we can deal with the numbers. That takes effort however, by default, we chunk. The price just abuses a common rollover point most people share.