DR400-180
By the look of it the Robin DR400-180 comes the closest to the mission requirement it will make the range, speed and payload requirement with ease.
Half the fuel burn of the Bulldog it won't do but as it can burn 91UL I would guess that you could get the cost per air mile down to about 60-70% of the Bulldog.
Other advantages of the Robin are very good short field performance and no VP prop to maintain.