CNC prototype machining is production machining with the quantity set to one, and that single change moves almost the entire cost into setup. Understanding which parts of the job are per part and which are per order is what lets a buyer ask for the right thing, and it is why two shops quoting the same file can differ by a factor of five without either of them being wrong.
Where the hours go on a CNC prototype
A CNC machining prototype spends its hours on programming the toolpaths, choosing and setting the tools, workholding, proving the first cut at reduced feed, then measuring what came off. On a production run these are amortised across hundreds of parts. On a prototype they are the job. A shop that already has similar work programmed will beat one starting from a blank screen, which is why familiarity with your kind of part matters more than the machine list.
Prototype CNC milling, turning or both in one holding
Prismatic parts go on a mill; round parts go on a lathe; parts that are mostly round with a few flats and cross holes belong on a mill turn machine that does the lot in one setup. Every additional holding is another setup cost and another chance for features on different faces to drift relative to each other, so a design that can be finished in fewer holdings is cheaper and more accurate at the same time.
Design moves that lower the price
Use the largest internal corner radius the function allows, because that sets the smallest cutter and therefore the feed rate. Keep pocket depth within a few times the cutter diameter. Avoid features that force a tiny tool into a deep cavity. Put tight tolerances only where they do work, and try to keep the tightly related features on one face so a single setup holds them.
What a prototype does not prove
It proves geometry, material behavior and assembly. It does not prove that the design is manufacturable at volume, because a machined prototype hides the draft, wall thickness and gating problems that a molded production part will have. If the production route is different, ask the shop for a manufacturability review against that route while the prototype is being cut.
Questions people ask about prototype CNC machining
Is CNC prototyping expensive?
Per part, yes, because one part carries the whole setup. Per answer it is often the cheapest option available, since it is the only prototype that behaves like the production component.
What tolerances can a prototype hold?
The same as any machined part: standard commercial tolerances come free, and anything tighter is an extra operation. Nothing about being a prototype loosens or tightens what the machine can do.
How many parts should I order?
Usually more than one. The second and third parts are cheap once the setup exists, and having spares saves a whole lead time when one gets damaged in testing.
Can a prototype be machined from the production material?
Yes, and it usually should be if the test involves load, heat or wear. Ask for the material certificate if the result is going to be quoted anywhere.