I was about the say melt off the existing brazed collar, but Doug beat me to it. Then I'd blacksmith the tube back down and braze the crack at the same time as I was brazing on a new pinchbolt fitting below the crack. With the new pinchbolt below the crack, the metal above and the crack itself would not be highly stressed.

I would repair this if it were my frame, because I'm a hobbyist and my time isn't money. But my cost estimate to do it for someone else would probably make them leave in a hurry. I don't like doing repairs, not as fun as creating something new, so my cost estimate is more or less intended to make them leave in a hurry. Although I'm more likely to want to do it if all the paint is off already. It's mostly the paint that makes repair work nasty. Oh and I'd want the seatpost out already, looks like it's still in there? I know how to get it out, but it's a bit unpleasant so I'd charge plenty for that.