Like many other replies, I keep track of spell components that have a gp value. I also keep track of spell foci, like the red dragon scale for aganzzar's scorcher, since it actually makes Eschew Materials worth taking and I'm of the belief that just about every feat out there should be worth it to someone in some situation. Plus, as a DM, it's great to remind players of this when I see there character sheet maxed out the gp limit with bracers, wands, and rings.