I dumped a lot of money into the Shadow and the Sable. I've dumped less in the Corolla, if you don't count oil changes.
One conclusion I've come to is that you can't count tire maintenance as part of maintenance. Well you can certainly factor in tire wear, and the costs of special tires, if your tires are not of a standard size and mounted on steel wheels. I'm talking about the cost of repairing flats and dented rims and steel wheels.
Flats can happen to anyone, as can dented rims. The frequency of such a happening depending more on what roads (or lack of) roads you drive on than anything else. There also pretty self contained damage. Sure there can be some strain put on the axle, but unless you ride for miles on a flat, this is hardly noticeable.
The conclusion of this observation is I will rarely consider getting a new car as opposed to replacing its tires. The car would have to be truly on its last legs. A busted hose that I would fix myself would put me into the "maybe its time for a new car" train of though before 4 new tires and a steel wheel.
The same is true of laptop batteries. I was recently given a G4 by someone. I spent $119 for a new battery for it. I did not think twice about the cost of getting a new laptop. Sure part of the appeal of it was "full size laptop with netbook battery life," and the cost comparisons to low end laptops go out the window if you only compare Macs to it. But laptop batteries are strange things. They die at irregular intervals. They are expensive to replace, but third parties often make slightly better performing batteries for the same price. This is especially true of older laptops simply because the third party will put newer more efficient power cells in battery cases designed for older laptops.
So my conclusion is always buy a new battery for your laptop. If you don't think you need a battery, buy yourself a desktop. They make small energy efficient ones that use effecient 2.5" hard drives and laptop CPUs.