The trick is to get a few earth-shattering, rip-in-the-fabric-of-the-universe type orgasms under her belt before "remembering" to mention how much it costs!
Besides, I wouldn't hesitate to drop that much $$ or even more on a new computer or a family vacation. I think keeping our sex life interesting is worth it. After 15 years of marriage, I think if you don't introduce some new stuff from time to time, there's a very real risk of getting bored. Fortunately, we are at a point in our lives where we can afford an occasional indulgence. I wouldn't do it if there was a question of paying the bills each month, but there isn't, so why not?