One is not permitted to derive benefit from a mixture of meat and milk. I'm curious -- would that prohibition still hold if the meat & milk mixture were inedible? I have no idea how one would derive financial benefit from, say, a rotting cheeseburger, but if one could figure out a way to do so, would it be permitted? Would it make a difference whether the meat and dairy being rendered inedible was intentional or not? (E.g., a cheeseburger that's gone rancid vs a cheeseburger that was made with soap mixed in.) Would it matter whether or not the meat & milk were inedible before being mixed?
Firstly, mixture is not the criterion for the scriptural prohibition. The scriptural prohibition concern cooking them together.
- Not cooked together See SA YD 87, 1, the words of Rema, a mixture in which meat and milk are not cooked together is prohibited__ rabbinically only, and for eating only, not for benefit (some Acharonim do not agree but the Shach concluded that this opinion is ruled.)
So a mixing without cooking does not lead to benefit prohibition.
- Cooked together: A second point is to know if the ingredient were rancid before the cooking or they was cooked in a good form and afterward become rancid. In the second case, following the Mishna in the last chapter of Temura (33b and 34a in folio of Gemara), >And the following are the things which are to be buried: ... [a mixture of] meat and milk; ... All things requiring to be buried must not be burnt, GEMARA: ... what is the reason? because the ashes of things which are buried are forbidden [to be used], ...
So it seems that a previously edible Basar Bechalav, even if it is now in a inedible state, is prohibited for financial benefit.
I noticed an interesting detail, If someone has at home Chomets who is not edible for dogs, he is not in duty to make Byur, but this Chamets is forbidden for benefit. Because Chamets is prohibited for benefice even after burning. But after burning it is mevoar (destructed) and no problem of owning.