Mattie's right that it's ambiguous, but the statement that "you gain a 10% reduction per class level" implies to me that it is

Final_Price = Initial_Price * (1 - 0.1*Class_Level)

Which leads to the original problem.

Mattie's assuming that they meant a 10% reduction applied each level on what it would have been before that level. I don't know that this isn't what is said, since it really does not state one or the other, but the "real world value" thing doesn't hold, since there is no standard in the real world that "a percent cost reduction per some quantity is always calculated by compounding that reduction as many times as the quantity." In the real world, the exact nature of the thing is actually described in detail so no one can mess it up.

Basically, this was never a matter of D&D's weirdnesses when it comes to multiplying, and always an issue of the order in which operations take place. Mattie's case seems unlikely to me since compounding the reduction is usually something that would be stated explicitly, and the wording definitely seems like the total percent reduction is calculated first, before applying it to the price.