>>1455758hyperdia's prices are accurate, NEX + Unreserved Hikari for comparison is 16,700 yen one way, though you could get a slight discount buying return tickets together.
Buses will probably be the cheapest unless you wait for Peach or some other LCC's special fare sale or the like. ANA/JAL do some 10,000 yen one-way fares deals for foreigners though on most all national routes though IIRC.