It's difficult to properly value them at this point. Six-ten years ago, they could be had for a few hundred $US, but some avid collectors blew that market up to the point where you can't buy a signed Kleenex from Buk for less than ~$600. So, we could help you verify whether the article is genuine and what the current market is, but even paying current market is more or less getting burned, unless you find a deal in some hidden corner. Keep in mind that there are various incarnations of the manuscripts. For poems, the earliest may be original typed copies with a signature, but more often than not, you'll find the carbon copy Buk retained with (hopefully) an original signature (many of the originals have ended up in library holdings). Later, you might find a computer print-out or copy with a sig. As for letters, they are generally restricted to originals (the genuine articles) and, unless they are a couple of words, very expensive and often highly-decorated in Buk's hand.
Good luck with your quest. I wish I'd had some of these on my radar 10+ years ago, but alas...