The value of an art piece is always subjective. The price (closest thing we have to the objective value) is determined by the buyers.

What other mechanism would there be? A committee? That's a bit nazi for my taste. A popular vote? Look at the election results to see why that might be a bad idea.

The theme is a bit touchy these days, especially in certain small country in eastern europe, where the new minister started to cut subsidies to the art she considers unworthy, obscene and politicized, in favor of art reflecting so called traditional values and national identity. The mere existence of the ministry of culture, established with the most noble goal of supporting art, creates this kind of potential vulnerability.