Your questions is somewhat vague. What is "small number"? What is "large number"? Making onesies is always going to be expensive, but when using PCB batching services at least you're not paying fixed upfront costs. Typically you are charged per in² of PCB space, with some extra for premium features like more copper, finish, etc.
However, to contrast really-low-volume (onesies) to really-high-volume (millions) you need to consider that there are different manufacturing techniques as well - where the board is punched, not drilled, and is made of phenolic resin, not fiberglass. This answer explains it fairly well.
This technique is specifically made for very high-volume production, and you just cannot use it for onesies.
David vs Goliath rant
On a somewhat related note, you may have noticed that, if you are small electronics manufacturer, on several ways you just can't compete with the big guys. Newcomers aren't welcome: they don't have the resources to set up automatic assembly lines, to use the really-high-volume PCB production processes, to twist the hands of overseas slave manufacturing, or even patent their work (with a single patent ~€10k, and that's for EU only; don't get me started if your gizmo has radio and needs FCC testing in the US).
OK, so you resign to making small batches of niche products, in this case likely the expensive PCBs may not be your biggest gripe. What startled me sometime ago was how costly custom plastic enclosures are. If you can fit your product into a standard box with drilled holes, then you're good to go, even though you need to tailor your PCBs to the existing enclosure. Big guys aren't constrained to that, since the $5k cost for a mold matrix is nothing to them. And they exploit that, because consumers value custom and sleek enclosures, they are viewed as "professional".
All in all, the industry begs for disruption, but I would not start with the PCB side of it.