Disease resistance is the quietest money-saver on a plant tag. It does not show in the first year and it shows every year after.
Disease runs along plant family lines, which is why it clusters. Fire blight moves through the rose family — apple, pear, crabapple, hawthorn, mountain ash, serviceberry. Cedar-apple rust needs a juniper and a rose-family host within about a mile. Plant several susceptible relatives on one property and they reinforce each other.
Resistance is not immunity. A resistant variety under enough pressure, in a wet spring, on a stressed tree, can still get sick — it just needs more to go wrong. That margin is the whole value, and it compounds over the twenty years a tree is in the ground.
It is worth most on anything that will be maintained, because a susceptible plant converts into labour. Pruning out blight in dry weather, sanitizing between cuts, hauling infected wood off site — that is a recurring visit, and it continues for the life of the plant. The resistant variety that cost a little more at install stops costing anything at all.