I do more or less the same thing. But I have a rule, and usually I stick to it.

When I started bike commuting again, it was probably early May, or maybe late April. I bought a jacket that was light and uninsulated enough to bike in, even up hills, but protected me from the rain and wind. Pretty much a necessity, because my commute is all uphill, and my other jackets are too heavy and warm. I justified this with "the money I'm gonna save by biking to and from work." I cheated, but made it work in the end.

Usually, though, I'll give myself a small budget, of about what I used to spend on gas. So at the end of the month, I'll have $X that I've unarguably saved thanks to my bike ... and some important thing I want for it. That logic is probably a little safer in the long run ( it keeps me from buying a CF cross bike with disc brakes ), but might also be easier to sell your wife on.