Not sure on exacts, but iv watched it done b4 - Its mostly labour costs. Done with a TIG to minimize localized heating.
I've seen both 'dots' every 1" along seams, as well as welding 1" 'seams', then 2" gap, 1" seams, 2" gap and so on.
I figure I'd give it a go myself, apart from making sure car is on dead level surface (to prevent chassis preload haha) and letting the work cool every now and then, I cant imagine there’s much involved.
In my case, I need to talk to my engineer about cages, to see if I can tie the strut towers properly to the A frame and firewall (re-route brake lines etc)
ne way