If the painting does earn you 30,000 at the end of the round, you'll wind up 4,000 ahead of the seller and 17,000 ahead of everyone else. If it only gets you 20,000, you're still up 7,000 on everyone except the seller, who is now 6,000 ahead of you. And if disaster hits and that artist places third or worse, you've lost money relative to all other players. One really poor bid could easily cost you the game. There's also a tremendous difference among the various auctioning methods.
Open auctions seem to be the best, since players can keep bidding the price higher and higher.
Open auctions for two cards at once are especially lucrative. Reserved price auctions are the riskiest, since if you name too high a price you'll be stuck having to pay it yourself. You have to keep a close eye on the number of paintings each artist has showing to make sure you can protect your investments. There's nothing quite so frustrating as when you pay for a piece that looks like it's going to be worth big money, only to have other players put out enough paintings from other artists to knock your favorite out of the running. In the fist auctions can be unpredictable.