Be sure to play with the fare classes and even routings.

When getting a ultra low fare the up fare is higher but the miles are the same unless you get a full fare (Y) ticket.

Sometimes getting Lowest Refundable will cost less as the up fare is lower and still paying the same miles as non Y fare.

But, also try lowest first class fare as I have seen this very close to the Lowest Refundable when adding in the up fare, then you don't use any miles.

Also, you might try booking one leg in first and the other in coach.