According to the wiki, 10 platinum coins
Make up your own price, unless you think 10 platinum is fair as it is.
To get money, I use a
crab engine connected to a few fish statues surrounded by a shallow layer of lava. The crabs spawn fish very fast and they very quickly enter my inventory. When my inventory is full, I sell all the fish I got, which gets me about 20 gold for each set of selling fish.
I could get the money, but it will be tedious. Then again, so is farming for such a rare item.