Its not exactly clear on what materials are involved. It looks like the top piece was designed with an interference fit, and pressed into the tube. I'm guessing aluminum (or is that zinc?) top piece and steel tube? It's possible to braze dissimilar materials together, but it takes some significant prep work. ($$$)
If it's aluminum to aluminum, it would be possible to TIG weld the two together. You could save some costs by removing ALL the paint from both parts within two inches of the joint before bring them to the welder. Obviously you'd have to remove the seat from the mount, and the tube from the bike frame first.
Have you talked to folks who do welding repairs? It may cost a lot less than you think. I know of one welder whose motto is "Tricycles and kitchen chairs have always been free..."