That $100 isn't going to help you play any better. It'll make you sound different, but it isn't improving your ability to play. The same can be said for buying a $30 guitar strap, quick-release pins for $20, a pedal for $80, etc...
For $50 (as of July 1), Capo _will_ help you play better, and using it regularly will make you a better overall musician. So will the other tools, which also cost $50.
My reasoning is probably quite similar to why they're priced the way they are--it is a seriously helpful tool to those that need it, and it is worth the money.
Now, why do I think someone would choose Capo over the other tools? I think Capo is easier to use, more well-integrated with iTunes, performs very well, looks far better, and is generally more Mac-like (see point 1 below). Any one of these reasons would be a valid reason to choose Capo.
In some respects, one could argue that I should charge more than my competition based on those 'better' points (see Eric Sink's software pricing article, for instance). However, Capo is a 1.0 product, and I need to build Capo's reputation, and a large following of users.
I hope this answers your question (and helps others out there looking for more discussion about pricing their own products)!
(point 1) The other apps are cross-platform, and have some odd quirks. The UI isn't quite right in areas, the keyboard shortcuts are strange (to me, anyway), and behavior isn't always what I'd expect.