If it's one of your favorite items then you haven't overpaid.
One of the guys I learned from told me "you rarely pay too much, just too soon". And taking a long view of collectibles I think he's right. (Of course, for some cards you might be a couple lifetimes early)
Plus there's only one of each one.
Storage of photographic stuff can be really complicated. There are some issues that aren't well understood, but research is catching up.
Essentially cool and dry is best.
Personally I'd make a small mat from some acid free paper to keep the emulsion from contacting the holder.
That being said, I have lots of old negatives that are still in the crummy boxes they were in when I bought them and they're doing fine. I've seen film collections in a lot of different states of storage and condition. Ranging from cool and dry that became junk to stuff in an attic that was beautiful.
The basics are here
http://www.pixmonix.com/tutorials/tu...lm-storage.php
With more details here
http://www.loc.gov/film/storage.html
If you follow the research link at the top of that last one it will take you to some technical papers. That's where most people hit the wall as far as interest goes. I find it fascinating. And the cellulose acetate plastic that's used is somewhat similar to cardboard, both beginning as wood pulp, so some of the science will loosely apply to cards as well.
Steve B