Back in
3e there was more of a formula for constructs (that was pretty easily discovered by reverse engineering the
MM constructs). It was based on size, HD, and CR.
Doesnt work that way in 3.5. There is no set formula when ya reverse engineer the ones in the
MM. And if you follow the rules for Craft Construct or creating magic items you wont derive the costs found in the
MM (much like a lot of the
DMG magic items it seems they were adjusted ad hoc...which is cool I guess).
Here's the "formulas" that do work:
(Price - ingredients cost) / 25 = XP cost
Ex: Flesh golem ingredients, 500 gp. (20,000 - 500) / 25 = 780 XP
(Price / 2) + ingredients cost = Cost
Ex: Flesh golem, ingredients 500 gp. (20,000 / 2) + 500 gp = 10,500 gp Cost.