My understanding is that Gatekeeper signing is free.
My understanding is that Gatekeeper signing is free.
The Apple employees that I am connected to were also surprised to find out that it wasn't free outside the Mac Dev program. A former Appler helping me dig around also noted that Apple never said it'd be "free."
If you can find somewhere Apple said it would be free, I'd love to dig deeper.
Also note this in my comment above: "...in the Grand Scheme of Business, it's just not that much money."
This is simply a mechanism for Apple to stay ahead of any worm-like activity and they decided to attach a barrier to entry ($99) and recoup a pittance while they're at it.
I'd also assume that Apple blacklists any certificate if the developer used some sort of fraudulent payment. I'd hope so, in fact.