May not like this answer but the truth is whatever someone is willing to pay for it. Not sure what you have in it but I can’t imagine getting much for it. Not too many people out there clamoring for a garden-variety 904. Even if it’s in perfect condition most guys don’t hold them in high regard for performance. They can be built that way but it’s a pretty small market for that type of application. Racers that use them would generally want a core and they would build them to their own specs., a general rebuild would not be useful in that situation because you’d want to replace the lighter duty parts.
That said - how is it set up? Good/race parts or just a generic rebuild kit? What year? Later ones (‘78-up) went to a lockup converter and are somewhat less desirable.
However, if it is a non-lockup 999 that’s a different story but you would have to know what those are and be able to ensure that all the internal parts associated with those particular units are in place.