Barn electrical, The price of copper is to expensive so can I use 4/0 4/0 2/0 urd to run between the main disconnect at the meeter over 180 feet to the 200 amp main breaker panel? And then run a separate ground wire in the conduit with it?

Yes you can.

As Stan said, yes, but why not 4 wire URD? Unless you already have 3 wire and want to use it up.