Minimum entry price to even compile a Windows Mobile app is $300 for Visual Studio Standard edition. Minimum price on the iPhone is $0 (XCode is free). The $99/year comes in when you want to run an app directly on an iPhone, and it also covers distribution. Windows Mobile, it's going to be whatever costs one would normally pay for web hosting/bandwidth.

As far as it being easier to develop for, it all depends on what you know. For someone starting from scratch, I'd expect learning C# and .Net would be no more easier or more difficult then learning Objective C and Cocoa.