I've been analyzing my spending along a single unifying principle recently. It's a good way for me to explain what was formerly a vague unconscious thing. And I've had this discussion with some of my friends and I've found them using my notation in conversation as well.
It turns out, if you want to be appropriately stingy, you want to optimize the ratio of cost to fun, such that you get the most fun for the least amount of money. You probably can't actually put a number to fun, because it changes over time, but you can kinda gauge it.
Either way, this enables a person to justify buying a thousand dollar bike or camera, because the cost/fun ratio is very high for those items. This also enables a person to decide that they'd rather purchase a book that will keep them amused for many hours as opposed to a DVD that doesn't have nearly the same amusement value.
Quantity plays a role in the cost/fun ratio. For example, buying a single book has a very high cost to fun ratio. However, buying twenty books, where ten sit in a person's to-read queue for months, results in you dragging down the cost to fun ratio of the whole batch. Similarly, buying every single mid-end digital SLR from Canon, starting at the EOS D30, the D60, the 10D, 20D, 30D, 40D, 50D, and 60D isn't nearly as fun as buying every other mid-end digital SLR... or even buying a smaller number of high-end digital SLRs.
These values shift over time, too. Five years ago, the cost-to-fun ratio of a new car was much more favorable. Eventually I realized that bikes had a much better cost-to-fun ratio. Especially because of how much better I feel after getting a lot of exercise... and the money I save. Digital cameras took a big hit once I realized how cool film cameras were.
Either way, here's a handy chart of where a few things that I've purchased or have thought about purchasing fall on the cost-to-fun chart: