by the sounds of it i would say you blew the freeze plug on the back of the block.. in order to change it you have to seperate the transmission from the engine to access it... i cant remember if you have to drop the flywheel also to access it... once into there replace it like you did the other one you already did... and yes if you run it that way you will keep dumping coolant out of it and without a constant supply the engine can/will overheat and possibly cause more damages.. as far as the cost it depends if you can and are willing to seperate the tranny yourself or with help of a freind.. or you can find someone who is more experienced if you are not... its a pretty straight forward job though especially if you have a repair manual to see how to drop the tranny if need be