OK no tomatoes! Of course one Birkin a year can be a fair deal. What is fair depends so much more than the number of bags per year. It's all in the attitude of both partners, the circumstances of the marriage, financial situations and other obligations, etc. (That said, I like yours, petpringles. You take interest in what your wife loves, and that imo counts a lot more than the number of bags.)
I have to say, though, that to me one bag a year may not work. This is because as you all know it is unpredictable when your dream bag might show up and how, and if you have to wait till next year you may never find it again. And I am not someone who buys just to buy, I try to be as strategic and reasonable as possible. So, for me, the solution is, I mostly buy my own bags. DH has bought me a few, and I am grateful when it happens, but i don't count on it.