If you look at your life like a business, you are effectively trying to determine how to allocate your scarce capital against a series of opportunities to minimize cost and maximize financial returns. Companies weigh these decisions all the time (e.g., when deciding on technology purchases many companies will weigh retrofitting existing devices for ~2 years additional usage, buying or leasing older models and buying or leasing new models), and these decisions are ultimately no different in your personal life. The only difference is that companies are driven by a series of rational investment imperatives and have to answer to their shareholders. We are driven by our emotions and often buy based on gut feel over hard, calculated facts.
In order to accurately determine this, I think we have to actually look at total cost of usage and ownership over time (I haven't read all his posts, but I think Jacob at ERE basically says something identical). All of the items we purchase have a function that they perform for us and come with a unique set of purchase, ownership and operating costs. When you take all of these costs together they form the 'lifetime cost of achieving X activity'.
This is effectively the argument behinds MMM's vehicle purchasing schematic. Instead of looking at a vehicle as a product, he views transportation as an activity and looks to minimize the all-in cost of going from point a to point b when utilizing a vehicle. To make this more practical, the MMM effectively says the average person drives 10K miles per year (ok less if you are actually moustachian), with an average active driving lifespan of 69 years (85 average lifespan - 16 years old when starting to drive = 69). The goal is to find the cheapest possible way, while still being safe of driving those 690,000 miles over the course of your life, including vehicle cost, repairs, insurance, gas, opportunity cost, etc. There is an optimal purchase price and model, length of vehicle ownership, and operating circumstances that will allow you to minimize the costs and make the correct vehicle choice.
This concept can be extended to virtually any capital item you will purchase, but with admittedly diminishing returns as you get into smaller dollar amounts of capital. In order to do the calculation properly I think you need to know the following: purchase price, expected usable lifespan, cost of ownership (repairs, insurance, etc.), operating cost (e.g., power consumption, gas usage), opportunity cost (e.g., lost financial returns from capital investment). I likely missed a few things in this list.
I'm too tired to give a highly detailed example, but I'm sure a simple excel spreadsheet could be built that would allow the easy calculation of different ownership scenarios to minimize cost for a given function that you want.