Welcome to comics - where the dollars don't make cents and the money doesn't matter.
But serriously tho - from what I know most of the time comics are a risky investment of peoples time and money no matter what the setup is. Its why most comics in the indie world are passion projects made by one person or a team of close friends. Most comic artists/writers work a day job to support their work, even the really successful ones. Sometimes that dayjob is freelance commission work, other times it's working retail, it just depends.
As far as paying for a print run - most people do a kick-starter or they find a publisher (or both...), or they use a POD service.
It's also worth considering that the cost per copy goes down the more you print.
This is also why selling merch and prints is so important - because the cost to produce them is so much lower than an actual book, and you can sell them with pretty high margins (you can also sell books with a high margin but that involves having a book >_>)