efficient
Explanation
This comic applies economic reasoning to sex in a humorously inappropriate way. A couple is lying in bed when one partner brings up the concept of economic efficiency, arguing that "free sex is impossible." They reason that whenever one partner desires sex more than the other, the more-eager partner should pay the less-eager partner an amount reflecting that difference in enthusiasm -- otherwise, someone is "getting something for nothing."
The joke plays on the economic concept of Pareto efficiency and the idea that every transaction should involve a price reflecting the marginal willingness to pay. The humor comes from the absurdity of applying cold market logic to an intimate relationship, treating sex as a commodity transaction rather than a mutual act of affection. The punchline -- "the market price turns out to be way more than whatever you have" -- is a burn implying the economist's partner is far out of their league, and also satirizes how economic models can produce conclusions that are technically logical but socially and emotionally ridiculous.
The comic is a characteristic Weinersmith riff on how economists can take a useful analytical framework and extend it into domains where it becomes comically tone-deaf.