Oddbod, the cost of commercially produced match ammo for obscure calibres is irrelevant - if adopted, it will be produced by arsenals for much the same as current calibres, material costs being equal.
6.5mm BR has the same rim diameter as the 7.62 NATO, and has a case length of 38.6mm, so be OK for magazine width.
Other carts which would work well are:
6mm PPC (rim .440", case length 38mm)
6mm-223 Rem (5.56 NATO necked up to 6mm)
.257 Kimber (.378"rim, 46mm case)
6.5mm TCU (as 6mm-223 above but necked up to 6.5mm)
6.5x39 (.445"rim - US army experiment necking 7.62x39 down as a potential 5.56mm replacement)