Stamps and baseball card prices have peaked and dropped a long time ago (except for a few rarities), computers are probably not at that point yet. There are few kids who collect stamps or baseball cards so down the road there will be fewer collectors to keep the prices up. There are plenty of kids collecting computers now, and down the road some of them will have deep pockets and will be buying rare items for $$$.

Eventually there will be books written that are price guide going by condition and grading for computers, followed by a peek in value of most machines and then a major drop for anything that is not pristine condition.

Anything that was popular will probably still be popular in 20 years and worth something. Prices for crappy machines will go up and down over time depending on fads (for example I think classic and classic 2 machines were crap, but some people who had one as a kid will want one, same with low cost machines that were used in schools).

Anyway the Color classic, SE and SE/30 will probably be the most collectable later compacts, the original mac will also be very collectable along with the high end desktop models people have always lusted after.