Sounds like you boys need to implement the policy my wife and I use...

Each month we each get an equal fixed sum to spend on our individual hobbies. What you buy is your own business. How long you save it up for a special purpose is your own business. If you run out of money that's your own problem.

Works wonders for helping you decide what you will and won't decide to spend your money on. Removes any possible source of arguments about what you've bought.

Kit