(many of the originals have ended up in library holdings)
Yep. The original poem typescripts are generally from 1968 or earlier, so when they come onto the market they naturally fetch the highest prices. Post-1968 original typescripts are very few and far between, but they have shown up from time to time. Though if you hold out for one of those you might find yourself holding out for many years. Or forever.
1946 - 1968: original typescripts
1968 - 1980: carbons
1980 - 1991: Xerox
1991 - 1994: computer printouts
But if you watch eBay for a few months you'll find carbons and Xeroxes. The selection ain't what it used to be, but they are out there. There's nothing on eBay right now, but watch
this guy. He's sold more than a thousand manuscripts in the past ten years and he still lists a few occasionally. You can always buy from a book dealer - if you're willing to pay twice (or more) what you'd pay in an auction (on eBay or PBA, etc.).
As far as placing a value on them, that's nearly impossible, because the sale prices don't really follow any logical pattern (almost two years ago I bought an original typescript for $520 and a Xerox sold a week later for more than $700). Personally I don't find any value in the Xeroxes or computer printouts beyond the value of the signature, but people still pay good money for those things. Pretty much the same price a 70s carbon goes for. So that being said, I would hold out for a carbon. But of course price will ultimately depend on how many people want it.
As far as verifying whether a manuscript is genuine, I don't think we've ever seen one that isn't. Though there must be some out there...