Welcome to the "old car cult and fetish club". My car is 24 years old, and I think it'll be good for another ten -- if I won't run into a dead-end replacing some emissions-control device. Smog-test last year was better than 2002, when I purchased the then-7-year-old vehicle. A radiator for my Trooper can run me between $90 and $150 at Rock Auto.
I'm not sure parts are getting cheaper. But I don't think they're getting astronomically more expensive. Between cheaper and expensive, there's always the problem that some parts just aren't available anymore.
For instance, I wanted to replace my hood-lock/latch and release cable. I've found two official Isuzu parts outlets -- they're still in the commercial truck business and they support their old lines of passenger cars. I found the items on their web-site; nothing said "no longer available", and I checked the "Add to Cart" buttons and checked out. Half hour later, I received an e-mail noting that the latch and hood-release cable are no longer available, and that my order was cancelled.
You might want to take a little time every other day or so over a month or two, and do a survey of new parts, used parts and sources. If something is already hard to find but you're still able to find it, you might weigh the pros and cons of replacing it now -- or just buying the part to keep on hand.
I'm not in a panic to buy parts for my old car, and I really need to discipline my expenditures in view of an option to buy a new vehicle. But these potential troubles run through my mind often. Otherwise, like the redneck mechanic from Thornburg, VA once told me -- "It's purr-feck! Purr-feck!"