My advice: don't keep secrets from your wife. (Unless you have a feet-on-keyboards fetish, that is.)
But seriously, unless you are spending the rent money on keyboards, your wife needs to be more understanding that a man needs something in his life besides work, eat, sleep. And $45 for a keyboard is hardly a budget buster.
Now with that said, spending money on keyboards is still a tricky situation for two reasons: (1) the "He doesn't really love me" problem: women hate it when you buy something for yourself, because they think, for some irrational reason: "If he really loved me, he would have spent that $45 on me." Now you can try to talk to her reasonably and calmly about this, and she may even admit that she knows her feelings are irrational, but she will feel it anyway. It's just how women perceive the world. I have no idea why. (2) The "clutter" problem: simply put, your wife thinks that your house is hers. And so everything you bring home is just clutter in HER house. Everything that she brings home of course, is an essential item, that your home cannot do without. I'm no anthropologist, but I'll bet the first caveman had a problem in that first cave when he brought home a few extra hunting clubs, and then he caught bloody hell, when his cave-wife saw that pile of clubs in her cave.
And finally, I have to LOL @ whoever said when women spend money on makeup, hair and nails, it's wasteful. I sincerely hope you guys are gay, or willing to experiment, because you are never going to have a relationship with a woman, until you realize that spending money on makeup, hair, nails and shoes (and handbags), are necessities, not luxuries, to a woman. If you keep your girl-friend/ wife well-stocked with that stuff, you can do almost anything. Oh you should learn to buy jewelry too, preferably Tiffany. Just my advice. You can continue to do it your way, because spending time with men is fun too. Personally, I like having sex with women, but everybody has to make up their own mind about that, I guess.