The 256 Newton had a short life as a factory loading. From 1913-1938 Western sold these cartridges. The Newton Rifle Co. failed in the early 1920's, and the manufacturers dropped the production for lack of sales.
A lot depends on condition, and individual cartridges aren't as desireable to Newton rifle owners, as a complete box would be, but I'd guess they are worth about $5 each, maybe more to the right person.
The 6.5 Mauser is not a highly collectable cartridge, and not much value on them.
