It’s true that there are alternatives to conventionally publishing, but in my discipline those options are seen as completely illegitimate if you’d like to pursue a tenure-track academic career.
You’re right to raise the fact that conferences have very real administrative costs. $300/person costs? Probably not. Comic conventions charge less and they pay their panelists and guests. Of course, they have a lot of paying guests who are getting something out of the whole enterprise — entertainment. They want to be there just to see the panels and meet the guests and walk around. Most academic conferences do not have such clear value to people who aren’t there for a CV line. It’s a networking opportunity, sure (and a paid vacation, if you’re tenured or grant-funded), but otherwise if you’re not presenting in some way, it’s hardly worth the expense for most people.
I think a reasonable middleground could be devised where academics who are presenting in, say, a panel are paid and non-presenting attendees shell out a small(er) registration fee. This might still require fleecing the poster presenters though — but then, no one actually comes to consume that content;. Most conference attendees who are in poster sessions are essentially paying for a small line on their CV (one that isn’t even well respected). It’s a weirdly overpriced commodity as-is, but I think it’s easier to fix than the publishing industry.