I have no problem with the question "Have they considered purchasing power". I could also come up with a dozen possible issues they might not have taken into consideration. But I would never argue that they havent considered them before I actually checked. And even though it seems reasonable to say

"The relative purchasing power of the money in the test is significant within cultures, let alone across cultures."

I am not convinced that is correct. A quote from the paper describing the study [1] suggest that the amount of money is not crucial

"Indeed, in the UG, raising the stakes to quite high levels (e.g., three months’ income) does not substantially alter the basic results. In fact, at high stakes, proposers tend to offer a little more, and responders remain willing to reject offers that represent small fractions of the pie (e.g., 20%) even when the pie is large (e.g., $400 in the United States). Similarly, the results do not appear to be due to a lack of familiarity with the experimental context. Subjects often do not change their behavior in any systematic way when they participate in several replications of the identical experiment."

The point of the article is that it is the norm to conduct studies where participants are selected from the same non-representative sub-population, and that this methodology is heavily biased. Rejecting this idea, because you find a possible issue with one of the many studies it is based on, seems like a really bad idea.

[1] "Economic man" in cross-cultural perspective: Behavioral experiments in 15 small-scale societies (http://authors.library.caltech.edu/2278/1/HENbbs05.pdf)