Art is filter that converts labour to beauty. İt consider luxury thing. True art is expensive. İt has reasons.
One makes art valuable is worktime on it. Artist may count hours how much he works effective on art's value. For example, artist finished an awesome painting within 40 hours. So, he count 15$ every hours. İt will cost 600$ only for labour.
Surely, materials determine also value of art. Artist may use silk or linen material, watercolor, paint. And he will add them on price. Maybe artist's style increase total price.
There is also difference in the value decisions for living artists and for dead artists. So, dead artist cannot create any more art. Their masterpiece is determined and they have higher price than living artist's. Because, their best or worst work may be yet to come.
Another reason makes art fantastic price is cleaning illegal money. We may think that İ never pay for so simple art which any billionare paid for millons of money. Actually, he saves his billions with spending millions. Then his money leaves country and comes clearly and consider legal money.