Here's what you'll get as a return on your investment: It's worth the asking price, but you don't need to spend time grinding for Souls in the Cemetery of Ash and around Firelink Shrine so you can afford it right away, as there's nothing in there you need immediately. At 20,000 souls it's pretty expensive - but the mystery just nags at you, doesn't it?
Early in Dark Souls 3 you'll notice the Shrine Handmaid sells a Tower Key.
