[zxlrmbjs] Messy capital gains

Purchase a good X as part of a bundle including other goods and services.  Sell good X later, not bundled.  How much capital gains was realized on X (for the purposes of taxation)?

Vaguely similar problems to problems for income tax if wages are paid as a bundle of monetary compensation and exchanged (in both directions) non-monetary goods and services.

Are their clean solutions?  If not, either unfair taxation or loopholes will abound.