From reading your comment I think this observation applies to your own understanding, not the gp's.

> whether the value of the money paid by tax payers to artists, equals the value of what they give to the tax payers in return. [...] one might wonder why they apparently are not able to sell their art for the same amount of money.

You might not see it but this is effectively equivalent to thinking only money has value, because you're describing a system whereby value is defined by money. Your dichotomy assumes anything that cannot be sold has no value, & anything that is sold is only as valuable as its price. The emergent conclusion from that formula is that only money has value.

It's worth noting that it also follows from this that value is defined by people with purchasing power. If for example the only cohort who value any given piece of art cannot afford to financially support the artist creating said art, not only is the art & the artist's work without value, but by extension so too are the perspectives, autonomy & - ultimately - the lives of that cohort without value.