cat? Cummins?Detroit?, why would you be leaking oil from a freeze plug???? IIRC the freeze plugs are for the cooling system(antifreeze)
A frost plug for the cooling system shouldn't be that expensive. You can even buy a frost plug kit pretty cheap and do it yourself. They aren't that hard to change.
if it is a freeze plug, most expesnive part would be fresh coolant, but that needs to be done every so often anyway