True developers with a purpose will not think twice about such a small fee. Nothing is really free is it? I mean the person putting out a free extension does it to be able to say they did it
$100 is not a small fee to everyone and Apple are essentially charging developers for the "privilege" of spending huge amounts of time creating extensions which are free to help Safari reach feature parity with its competitors.
For someone who only wants to develop extensions, the $99 fee is not worth it and they'll just redirect their energy to Chrome and Firefox. It these developers want to be able to say "they did it", they'll aim at the 60-70% combined marketshare of Chrome and Firefox rather than the 5% share that Safari has.
This is just another example of the supreme arrogance of Apple - that they think they can charge this and developers will still code for their single-digit-market-share browser is hilarious. They have totally misjudged the balance of power with this decision because for once it isn't with them, its with the developers