Two goods having about the same price means that, by and large, people would exchange these goods for each other. It means they have roughly the same value.
Now, if you want to "put a price on art", just tell me what you would exchange for a given painting. If you would trade a bunch of soup cans - that's what it's worth. If someone is willing to spend a million on it - that's what it's worth.
The nominal value we print on currency is arbitrary. If you added a zero to all of our dollar bills, our economy, after some hiccups perhaps, would roughly settle for good A and good B having the same price after if they did before.
