You might get more replies if this thread were in the "Freelancing" Forum. Maybe a mod will move it.
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I'm getting back into freelancing after a long time away, with moves to two other states between. So I'm not part of a network here, and a lot of what I like to do (video scripts) involves who you know.
Last fall I met, by chance, a video producer and gave him my card that said I was a writer. He said, "Oh, I'm looking for a writer." That led to my first job with him, in which I wrote a script for free. We met to go over the project, he later emailed to ask what my rate was, and I said, after considering, that I'd do it for free because I was trying to re-establish myself and also because I liked his project and I knew he had a small budget.
Ok, so did that, everyone was happy, got a new script credit to my name. Fast forward a few months and he emails me with a new project with a generous scripting budget. He wanted to offer it to me first. Yep, of course I took him up on it, gave him a good product, and I know he'll use me again.
Between the two scripts for him, I made about the going rate, I think (not really sure of what the market rate is here in my new area).
If your pro bono work could lead to future paid gigs, then I'd say go for it, if it's not too extensive. Just do as good a job with your giveaways as you would with paid work.