I think there is a lot less black magic than you think. The overwhelming majority of the time the small price fluctuations are in response to changes in the price by 3rd party sellers (the ones selling the same item in "new" condition).

just check for yourself at http://camelcamelcamel.com/

They virtually always undercut the 3rd party - which always begged the question: why do the 3rd parties even bother trying to sell on amazon?

Actually if you think about the kind of testing you're talking about, it doesn't make a whole lot of sense from a human psychology perspective. You don't look for a camera and think: "This should cost $123.76" and anything less will be huge turn off. There are probably more drastic things at play, like if the price ends in .99 or if it's more than a hundred dollars, or maybe people don't like certain price numbers for some completely random reason.

They probably have some internal database of the average (across all inventory) of the rate-of-sales vs. the listed-price and the algo just looks for a local maximum around the price that is a bit lower than the 3rd parties.