I don't know that I'd go so far as to have a monthly budget for knives. I have a paypal account that I use for my ebay activities (mostly selling) and whatever cash is in there is seperate from the family money. If I want more knife money, then I go to garage sales and find stuff to sell on ebay. It's my cash to do with as I please. My wife's fine with that. I've financed most of the tooling for my shop the same way.
I also agree with the no knives on a credit card philosophy.
If your wife's not into purses, find out what she does like and occasionally buy her something as a surprise. I strongly recommend avoiding the DEBEERS thing mentioned above unless you're in a much higher tax bracket than I am. It's hard to do on a regular basis at that level.
This strategy has worked so well for me that at the Solvang show last year I had been agonizing about a Hibben custom and my wife kept telling me to just buy it. I hemmed and hawed and looked around some more at other booths and when I finally went back to buy it, it had been sold. As a consolation she made me buy a small unique decorated Sebenza with a blue Agate inlay. It wasn't until some months later that I found out it was her who had bought the Hibben when I wasn't looking when she gave it to me for my birthday!
John