This is a question- perhaps the only question- I care about right now. I'm obsessed with the idea of growing an Audience for an Artist in generic terms- without any aesthetic or commercial criteria, because... an Audience is an Audience, whether it's for the Insane Clown Posse or Julia Holter- Audiences of equal numbers are equal, it doesn't matter what they like just that they all like the same thing.
It's clear to me that growing an Audience for a particular Artist or work from 0 to 100 to 1000 isn't particularly mysterious. It's your personal friends, or the personal friends of the people you are working with. From 1000 to 10000 requires some gatekeeper attention- record label, booking agent, critical acclaim, good manager or an incredible work ethic and (these days) internet savvy.
Once you are in the 10,000's though, some different dynamics take over. Specifically, you are now in the realm of "Mass Media" even if you are just some guy strumming a banjo in Phoenix on Youtube. And, the number one principle of mass media is that a POTENTIAL Audience is available for purchase.
If you have an Artist with 10,000 plus fans, and a combination of a product in the marketplace being promoted and touring surrounding it, you can purchase a potential Audience of 100,000. From there it's just a question of whether you can afford it, and how long it will take to reach an Actual Audience of 100,000 (if ever.)
Bridging from an actual Audience of 100,000 to an actual Audience in the millions is where the big boys play. You can land a potential audience of 100,000 for not that much, but trying to get a potential Audience of million or more in a set time period (a week, say)- that takes either ALOT of money- thousands-tens of thousands- and ALOT of connections with professional music industry people or, most often, both.
You could live your whole life as an Artist and never have an actual Audience that cracks 100k- supporting yourself and everything, that is where efficiency comes into play and there is a difference between the Artist with an actual audience of 25k who controls their products vs. an Artist with an actual Audience of 250k who doesn't. The former keeps more of the income generated, the later does not. Huge difference 10, 20 years down the road.