You shouldn't have to prove that you're "serious about development" by paying money to write and distribute software. How many developers started as hobbyists?
Charging $100 just to be capricious is not a good move and is certainly not a good omen for OS 10.9 "Tabby" wherein you can be almost certain they will remove the option to run unsigned software (for your own protection, of course! You don't want to pay Apple $100? What are you, poor? The computer cost $1000! $generic_strawman_argument!)
You don't need to pay ANY money to write software for the Mac. Xcode is free. You don't need to pay any money to distribute software for the Mac. Distribute it through your own website. You need to pay to sell through the Mac App Store. I also presume you need to pay if you want it signed. Well that's a privilege. It helps you prove to potential customers your app is safe. You benefit from it so you should have to pay for it.
If you develop an app with the purpose of selling it on the Mac App Store for profit $100 should not be a problem for you.
If you want to distribute it yourself, go ahead. Apple is not charging you.
Charging $100 just to be capricious is not a good move and is certainly not a good omen for OS 10.9 "Tabby" wherein you can be almost certain they will remove the option to run unsigned software (for your own protection, of course! You don't want to pay Apple $100? What are you, poor? The computer cost $1000! $generic_strawman_argument!)