Unless I'm mistaken, that well can give you any of a random set of things like money or experience points but it can also turn you to stone or eradicate you. The trick is that with one of the high level spells you can cast, it gives you invulnerability to eradication and maybe the other conditions which means you can just sit there and keep clicking on the well to your heart's content and rack up the money and experience points. It's a total exploit.
Cast "Protection From Magic" and keep wishing at the well. Everytime you activate it, it like gives either: 10k gold, experience, skill points, drops your armor class to 0 (temporary), stone, eradicate, death(?) So what you would want to do is to use it on your cleric first, so that they can cast lvl60 Protection From Magic, for longer protection. It will protect you from the bad ones 60 times before it goes away.