Both BobRay and Bruno Perner (to name just two) create exceptional Extras and are supporting users of those Extras on these Forums. Those Extras transform both how we use MODX and how we create websites, yet are "free". I don't know how sustainable that model is and yet they still continue to crank out new Extras and improve features of existing ones.
Interesting topic.
With respect to the sustainability of the model -- if I needed the donations and purchases of my extras to eat, I'd be heading to the food bank about now.
My experience with support is that if the extra is solid and the docs are good enough, there's almost no support necessary. I'd say that about 90% of the interactions I have with users are about feature requests.
For commercial extras, pricing is definitely a horror. There's really no way of knowing the optimum price for an extra. 500 sales at $10 is a lot better than 10 sales at $30, but maybe you'd have the same number of sales at $30 that you would at $10.
I thought everyone with a MODX site would want SiteCheck and that $25 was a reasonable price for an unlimited-site license on a tool that could save you time and trouble. Sales have been relatively weak, but I really don't know if the price is a factor. One thing is for sure, you can't price things by how long it takes to produce them. I worked on SiteCheck (to the exclusion of almost everything else) for at least 6 months and on MyComponent (which is still free) for well over a year. I'm sure Redactor also took a lot longer than it's price would indicate.
On the topic of SimpleCart, I would expect that any full-featured, reliable e-commerce extra would be a total monster to write, test, and create docs for. I definitely envy them their ability to charge people by the month.