A $35k project came in once, and I said no.
The budget was there. The project was big and exciting. But we weren't confident in the tech behind it, and I wasn't going to take someone's money on a "we'll probably figure it out."
So we passed. Hard to do, but it was the right call.
Around after that another client came with $15k , and that one was also slightly beyond us. The difference was, I could feel we'd get there if we pushed. So we took it, stretched, and pulled it off.
That's the whole skill. Knowing when "we're not sure" means stretch, and when it means walk away.