One can expect the 760MP mobo to be more expensive than the 760 mobo, however the increase in cost should not be anywhere near a 2:1 ratio. Plus you gain additional savings from decreased port usage on switches/hubs/KVM and decreased cost in memory/HDD/FDD/etc...
The low-end Mustang variants (Palomino and Morgan) will be available by then which may turn out to be a better choice than a Duron depending on what changes are included in the Mustang core.