If you've been interested in working on your own motorcycles for any length of time, then chances are excellent that you've already watched prices change over time. Even if you've only been changing, say, your oil and brake pads over the past handful of years, you've probably had some opportunity to watch prices change on these regular maintenance items that every rider has to attend to if they're going to take good care of their machines.
Of course, the deeper you get into any given bike, the greater the opportunity you will have had to see this. From small things like screws to more specialized things like specific gaskets, or carburetor boots, or even full assemblies, pricing (and availability) will vary wildly over time. And this isn't a hard and fast rule, but generally speaking, the further away from when a thing was new that you get, the more expensive and difficult to find parts may start to get.
There are exceptions, of course. I love my vintage Hondas for many reasons, but one definitely has to be that I'm not made of money. And comparatively speaking, because Honda has had a tendency to use a lot of the same parts across multiple models, there's often a good chance that you'll be able to still find most parts for even my 40-year-old V4. Are there certain things that are more difficult or expensive to find than others? Of course, but it's a lot easier on the wallet (and your headache medicine supply) than some things. It's all relative, you know?
