On older houses, the downspouts often run into a pipe in the ground instead of onto the lawn. When that pipe clogs or breaks, the water backs up and overflows the gutter above it. We check where each downspout goes when we inspect.
On streets with big trees over the houses, gutters fill with leaves and seeds every fall. A gutter that is too small or pitched wrong overflows sooner. We size each run for the roof that drains into it.
So we do not price gutters from the curb. We look at the roof edge, the board behind the gutter and where every downspout goes.