I must object to trying to apply "basic economic theory" to sex.
Sex is not some kind of factory-produced commodity. Not is it an objectively measurable service, like painting a wall or producing an artifact. If you want to compare it with anything else that is bought and sold, compare it to art. Its production and its assessment are intrinsically idiosyncratic. There can be no rules for "value assessment", and any rough consensus that emerges is temporary, a succession of fads that exhaust themselves as they become "common" and "old". And sex, unlike art, cannot be preserved and resold, it is a performance thing. Each instance is one-of-a-kind to the participants even if it can be fit into "types".
Economic theory in itself (the reduction of social like to "the market") is wrong, bit it is especially wrong when it gets applied to activities such as art - or sex! "Sex work" is not that simple. It is not an industry and the tidy rules of supply and demand do not apply. Side by side with the "sex market" is "free sex" for the offering, also: what industry produces stuff that is usually, mostly, given for free everywhere?