The actual equation, including the second drop, is 625 * (1 + 0.2/(1 + e^(0.02*(x – 630))) + 0.1/(1 + e^(0.2*(x – 1760)))). Determined experimentally, with help from Bruno's statement that it's the sum of two logistic curves.

Effectively this means that there is 30% bonus over Bazaar value at first, which then begins declining down to a 10% bonus, and then the 10% bonus goes away much later. It's not so strange that the centers of the logistic curves are in odd places when you consider that the values were picked based on giving the full and partial bonuses up to certain points: i.e. they were picked to put the top of the curve at a certain point.