>>Basically came down to some variation on "I don't ask for a lot (she doesn't), but I want the diamond ring."
As some one newly married, I have observed the following.
1. See <something>. Demand for it.
2. When rationally explained why we can't buy it.
3. Simply state.. "I don't ask for a lot, but I want the <something>."
4. I buy it.
Did you try modifying step 2 to something along these lines:
"Great idea, I never thought about it, we really need <something>! Problem is, this particular model of <something> is not the best and will not do <something that something is supposed to do> very well. In fact I remember reading bad reviews about this. But fear not! We'll go home, spend some time reading review sites and will find a really good version of <something>."
This will buy you some time, and might even look that YOU want it, which might even make it less appealing to your other half. Now, find a really expensive model of something, but at such price point that the rationality wins, even in someone who usually does not listen to your arguments (step 2). YMMV, but sometimes this tactic works out quite well.
Interesting assumption there that he is buying her something. Agreeing as a married couple whether to spend money on something doesn't indicate that it is his or her money, the discussion implies that money is shared. For all you know if could be all her money in terms of income.
As some one newly married, I have observed the following.
Repeat steps 1 - 4 endlessly.