Pricing would depend on whether the car is a real factory 340 car.
The fender tag, dash Vin plate, drivers door VIN decal for a factory 340 car should read: VS29H1BXXXXXX
A real factory 340 car in need of a full resto., but that is running driving, with all the factory parts in place, may go for anywhere from $1,500 - 5,000, depending on the condition, #matching drivetrain, etc.
The more rust repair required on the body/chassis, the lower the price.
There have been some fully restored 1970 - 1971 340 Dusters, recently selling on Ebay and other Mopar sites, for over $20,000.
Restorations cost time and $$$.
If you only looking to have a small block Duster for the street/strip, then buying the best body / overall condition slant 6 or 318 car is a better starting point, from a cost $$$ standpoint.