Is it appropriate to include dollar amounts for awards granted when applying to a math PHD? This question includes REU (research experience for undegraduates) grant, travel grants, or prizes awarded. I've seen a friend's CV and he seemed to do this. I was wondering if this is to be expected or if one should refrain from posting the amount awarded.

Perhaps it is worth mentioning that participation in an REU (and the support received) does not really belong in a "grants, honors, and awards" section, or at least that is my impression.

There is a similar question on this website, but I'm asking specifically about  applying for graduate school and what is culturally accepted among the mathematics community in the United States
It is entirely your choice -- doesn't hurt, but not necessary either.

Readers of grad school applications generally have a decent idea of how much REU or travel grants (a couple of thousand dollars) and how much departmental awards are worth (a few hundred dollars, typically). So not stating a number is not a big deal. Stating a number also doesn't hurt: if it's within the range a reader would expect, then there is no harm done, space is cheap; if the dollar amount is surprisingly large -- say, a $5,000 award for your work, then that's useful knowledge and should have been on your CV indeed.