I beta for free, but not for everybody who asks. I limit myself to genres in which I'm well-read, and to writers whose work I've seen in smaller amounts, like a chapter at Share Your Work. And, I confess, I won't beta for someone who's rubbed me the wrong way, even if that's totally unrelated to the work which needs beta-ing. It's a major investment of my time and effort, and I don't hand that to people who have managed to provoke my temper.
I'm fairly fussy; if the manuscript has not been made as good as it can be before I see it, I'll only do a chapter or two, then return it with notes on what it needs. I don't have time to teach grammar and spelling.
What do I get from it? A good read, usually. The chance to contribute in a small way to making it an even better read. The knowledge that I saw it before it reached print. Confirmation that some of my input caused changes. And sometimes--not always, by any means--a beta read for my own work.
There are people here who beta a lot, and they're to be thanked as generously as the authors can manage. I've learned something from every beta I've had.
Maryn, not quite too old to learn, but pretty close
