I am part of a prototype preservation and research group. Several of us have asked about most of these on other forums, but I am trying to cover all bases.
We will pay handsomely (US$1000+) for alpha builds of Soul Reaver. At least three are known to exist, from early in 1999.
We are also interested in prototypes of the following, and willing to negotiate on prices.
We have quite a few already of the first three types, fewer of those below, but are very interested in things we do not already have, even if we already have many builds for it.
* Any Crystal Dynamics games from mid-90s to mid-2000s (esp. Whiplash, Mad Dash Racing, Akuji, and Gex)
* Any Silicon Knights title
* Other games from the Legacy of Kain series (Soul Reaver, Blood Omen, sequels, etc.)
* Dead Space (esp. the unreleased OG Xbox prototype)
* we have at least one prototype for each game in the series, but other versions are of interest
* Aeon Flux (PSX) - except for the mission 2 prototype, which we already have
* Dead Phoenix (GC)
* Elric (PSX)
* Stormbringer (Dreamcast)