If I have a tightwad client paying big bucks for my services AND pays for my travel, I'd be happy to pay for the upgrades. Of course, I'd ask once if they would spring for them. Worse they can say is no. I imagine they would also appreciate the fact that I asked and didn't just forward the bill.

However, if I get caught pulling a fast one and billing for upgrades, I could be without a paying client. Then I really couldn't afford the upgrades, much less a plane ticket.