Like TB, I have worked on bikes for many years, as a dealer tec, and I have found that the cases can most times be welded "in situ", if you can find the guy to do it.The exception is where a crack is intersected by a glue/gasket joint or as TB mentioned, if the crack gets into a precision bore or journal.
It takes a bit of prep work,tho. Sometimes the bike/quad has to be flipped on its head or suspended in a specific way to be able to keep the joint clear of contaminating oil. Still cheaper than a teardown, and if my welding tec says he can do it, I trust him.