they're really only around $20-25USD. it's more expensive than an app...but the quality is loads better...and the fact that it is a widescreen angle, means you can get more into the picture without having to stand 50 feet away.
here's a video of my lens in action:
theres a link in the description if you wanna buy one...you can still shop around for better deals/lenses though. there's also a link to my blog for some shots i took with it paired with the hipstamatic app.
also...if you do end up getting a physical fisheye, most of them are magnetic....i'd suggest superglueing the metal ring it comes with to an IPHONE CASE, and not to the iphone itself.