Question

What can influence an artwork to become valuable?

Answer 1

there’s multiple things that can impact a paintings value.
Anything by a famous painter will be valuable (if Picasso signed a napkin It’d be worth millions).
another big factor is fame, the Mona Lisa wasn’t price less until it was stolen and got a lot of publicity.
Popularity, if no one likes a painting then no one is going to pay for it.
Rarity, if there are millions of the same painting the value will decrease because the painting would be easy to get.
When it was painted, lots of paintings values increase as they gets older, styles change, they becomes rarer, they become a part of history.