I think your idea of cost per month is good for things you use often. One thing I've been thinking about lately as I've undertaken a medium-sized kitchen renovation is the cost of ownership of tools. Since I'm not a professional I can't take a cost per month approach since I may not use the tool for many months at a time.
Instead I've been thinking about two things:
1. How many hours of operation can I get out of this tool? If I amortize those hours over my lifetime, will I ever have to buy the tool again? In this case a mid-range tool may be an A+ for me, but a B- for a professional.
2. What is the lifetime of a battery-powered version of the tool? I'm avoiding battery-powered tools (with a few exceptions, I'm looking at you power drill) because I'm concerned the batteries will fail before the tool does.
My ideal situation is that I buy these items once and never need to replace them. As an amateur, I should be able to do that without buying the most expensive tool. As a professional, I probably already have preferences and a strong opinion anyway.