I might be wrong, but I'm not sure that you can fit a mobile drilling unit in a Merc. Check out the size requirements in the dock.
btw I found this economy guide which is very interesting
http://www.the-commander.com/x2economy.htm
also a factory building gude.
http://www.the-commander.com/x2factorybuilding.htm
I'm currently transporting my silicon mine to Three Rings.