If I was recieving all the money myself for Natural 20 Press .pdf sales, I could make a decent living at it - so if you're doing all the work yourself, and you can get good sales figures, then, yes, it's possible.
As you bring in other people, though, it becomes less viable (but, conversely, your project becomes more professional and so probably sells more - I have no figures to show how Nat 20 products would sell if they were all done solely by myself, but I suspect they'd be considerably less).
I'll be straight - each Nat 20 product nets well over $2000 in the first month, and that's when we sell it very cheaply ($5). I have no doubt that I could increase that price, and may try it for future products (an increase from $5 to $6.95 made no difference at all to sales of Wild Spellcraft)). Of coure, a lot of that goes to paying people (and Nat 20 is very generous with that, given its "community publisher" role), so I personally make much, much less than that.
All in all, it doesn't compare to print products - although I should find out firsthand when I see the sales figures on the print version of Wild Spellcraft.