Not that it matters to me personally, but I wonder how historically accurate that is? I think of that kind of hardware as being more the product of an organized army, or city. Were roving bands of mercenaries building war machines on their own, and renting them out? Ballistae R Us?
Siege engineers typically accompanied an army , but they sometimes came from abroad. Think of it this way you have skills and there is an army that is in need of those skills. So are you gonna be jobless or get mere pennies or go for the gold.