http://www.shootersproshop.com/ammunition/shopby/f/cartridge/26%20Nosler/isAjax/1.html26 Nosler ammo nearly half the $90 6.5-300 Wby ammo price.
The 26 Nos is bordering overbore, the 6.5-300 Wby is more of the same, more powder for a little more velocity.
Someone mentioned 6.5-378 Wby. There are some huge over bore 6.5 wildcats out there, not that impressive for the powder burned, IMO.
The 6.5 GAP does make a lot of sense.
I have a 6.5 Creedmoor, a 6.5x55 and a 26 Nosler. I think that about covers 6.5's for me.
DF