Make it free to learn Windows Phone 8
100$ is a lot for developers in 3rd world country like me. I have to think a lot of times before I even decide to buy Lumia 520, Lumia 920 is close to twice my monthly salary.
Microsoft is already lacking in developers, so why not make it easier for people to embrace the environment?
If you want to charge, charge when developer want to publish their app in Windows Store. For personal learning and debugging purpose, why the fee?
Simulator is great, by it's not the same.
Chris Gonzales commented
Learning and developing on the emulator is completely free. There are also plenty of promotions to make your money go far. For example you could purchase a year license for $14 about a month ago. Also, if you are a student you can get in free via Dreamspark. Through the Nokia developer program you can get Telerik tools, a year developer account, and some others for $99. Through the dvlup program you can get a smartphone etc. for completing development challenges. If you attend local MS sponsored hack-a-thons they give away smart phones.
There are plenty of ways that you can get into windows development for no money at all. The most important point is that you don't have to have a developer licence to build and test on the emulator.