They don't provide “good prices” compared to traditional printing for any kind of volume, at best they provide good prices compared to common retail prices for traditionally printed work, but a publisher using them still has to amortize fixed non-printing costs across the print run, they can't just sell for the printing costs and even break even.

Those kind of services do make very small print runs possible at lower prices than would be the case for traditional printing at the same time (because they have lower fixed and higher per-unit costs than traditional printing), but they don't represent a reduction in overall printing costs.