I do a budget for photography every year. Right around New Years is a good time.
It is a carry-over from when I had a studio, but you can do the same thing. Equipment, travel, magazines, classes/workshops, repairs, etc. It helps me throughout the year when temptations crop up. If it is something I budgeted for 6 months ago, it is pretty easy to move forward without feeling guilty. If not, I know that I will have to give up or shuffle something else on the list.
The only difference here is that I don't look at the outright cost of a camera (or studio flash system, etc.) I look at the net "Cost of Ownership", basically the depreciation that I expect for 1 to 2 years.
In this case, you might expect about $500 to $600 the first year in depreciation, maybe $300 to $400 in eth second year. That might be a bit high at this price, as this swill still be a great camera in 1 or 2 years.
That assumes you have the cash to purchase. But cost alone was never a factor for me.
If I could get a really good deal on something that I could later sell at the same price, that was a no brainer. Not so much a camera, but high quality Canon and Nikon lenses, for example, hold their price really well. Especially if you buy a high quality used lens at a good price (buying new you can expect to lose a certain percent of course.)
That also makes the upgrade decision a lot easier. Even your older body will depreciate a certain amount. the question is the incremental depreciation on the new body versus the old, not the outright purchase price.
That is what makes the lower end cameras such a steal now (at least 6 months after introduction.) The depreciation on a Canon T4i at $600, or a Nikon D3200 at $500 (list is $595, but I bought a new one a while back at $500) is only about $100 a year.
That is an incredible bargain, especially compared to shooting film! Pro color 35 mm film used to cost me about $0.33 per frame, so 300 images is break even! Medium format 6x7 used to be $1 per frame!!! That makes a camera like the D800 very easy to justify.