Here's my trick, all my riding buddy's use me as their personal mechanic; which I enjoy and my kids like to get their hands dirty too tinkering with allen keys etc. At any given time I have at least 10 bikes on hand and my wife has no idea that 8 of them are mine. If I bring in an unfamiliar ride, she doesn't even flinch now, just checks out the garage to make sure she can still park in there free from the ice and snow; or sun in the summer.
Another plan is to sell the old bike and some other stuff on eBay and when you come up short a few hundred or so, sit on the couch and watch sports during the opportune times for Oprah and E.T. and such........... she'll ask you "what model bike did you want again".
As far as my first trick goes, I don't condone dishonesty toward your wife. If my wife asks if that's mine, I own up. The last time she asked, I had already had the bike for about 6 months. So I said, yeah it's mine, it's been here for 6 months (no money missed means no money to argue about)
Here's my last angle, I have a shaved head ............ $30.00 once overy 3-4 years for clippers vs. $65.00 every month for hair "styling" which requires an imediate wash upon returning home to "get all the crap out" That adds up to just over $3100.00 every four years. Your bike was an '04?
It's 08 now.