I've got a Star Trek VI Enterprise kit that a friend asked me to sell on e-bay for him. I just gave him $5 for it because looking at the prices they sell for there, it wouldn't be worth the effort of listing it. I'd probably lose money on the shipping costs.
I see the AMT figures, particularly the Scotty, for sale all the time at around $5 to $10. I've even seen all the ST figures sold together as a bundle for around $40 recently.
For the moment at least, the newer Star Trek kits are what's hot. Same goes for Star Wars - the Finemolds version of any SW kit goes for a lot more then the older versions.
As a good starting point, I'd go look on e-bay. Watch auctions for kits you have, and see what the final prices are. That should give you an idea where the market on any given kit is right now.
For the most part models aren't really a very good long term investment, financially speaking. The base of the hobby is shrinking, and when you factor in future inflation and increased shipping costs, you are very unlikely to be able to retire by selling your stash in 40 years.
If you really must try this, then rarity is the key. Kits that flooded the market will probably never be worth much, while limited runs will do better. The real thing to look for is something that's very iconic but had a limited production run - especially if the franchise is unlikely to be revisited anytime soon, or if the cost of re-popping is unlikely to recoup investment costs.