Gnomon,
Certainly how much a DT conversion costs to do is affected by many things. Since each single trigger design is different, there's no way to apply a sweeping statement about what it would take to convert all single trigger guns to DT. I have no idea what differences are in a single trigger H&H gun vs their DT. It could be very different on significant parts or it could be just like these American gun single trigger designs.
On a gun like that factory Ithaca single trigger shown in the second picture, it would require either replacing the floorplate or welding it up and either replacing the sears or straightening them. Also some safety parts would need to be added.
But for the most part, double triggers are about as simple as it gets.
