The shown sale price is second highest bid. It sounds simple, and it was a simple way to implement proxy bidding. In practice, it gives your brain some exercise : (
[Yes, it is on the "to replace" list, but there are lots of higher priority tasks.]
Our intention was that you would bid what you thought the domain was worth and forget about it... unfortunately, there are trust issues with that, and most customers keep doing the incremental bidding thing.
Thanks V and S for the explanations, and for your patience Frank.