A machined prototype is the only kind that behaves like the production part, because it is made from the same bar of the same alloy by the same process that will make the rest. That is why it costs more than a print and why it is worth it as soon as the question you are asking involves strength, stiffness, heat or a fit that has to hold under load.
Why a one off is expensive and what it buys
On a single part the setup dominates completely: writing the program, fixturing the blank, proving the first cut and inspecting the result are the same work whether you make one or fifty. Nothing about that is waste, and it is why the second part off the same setup is a fraction of the first. Ask for a price at one and at ten and the shape of the shop's cost becomes visible.
The features that cost the most
Deep pockets with small corner radii force a long thin cutter and a slow feed. Features on five or six faces force extra setups. Tight tolerances on features that span two setups force a fixture. Thin walls force light cuts. None of those are unreasonable to ask for, but each has a price, and a designer who knows which ones they are can usually give one away without losing function.
Getting the lead time down honestly
Most rapid machining offers work by holding machine capacity and standard material in stock, so the fast route is available when your part is in a common alloy in a common size. An unusual specification means the clock starts when the bar arrives. Say early what can flex, because a shop that may substitute an equivalent grade can often save days at no cost to function.
What to send with the order
A model, a drawing that says which dimensions are critical, the material specification, and a note about what the part is for. That last note is not a courtesy: a machinist who knows a surface is a seal face will fixture and finish it differently from one who reads it as another face on a bracket.
Questions people ask about prototype machined parts
How fast can a machined prototype arrive?
Simple parts in stock material commonly ship within a few working days. Complex multi setup parts, certified material or an inspection report will add time.
Is machining or printing better for a prototype?
Print when the question is about shape and fit. Machine when the question is about strength, tolerance, temperature or wear, because those depend on the real material and the real process.
Why is the second part so much cheaper?
Because the programming, fixturing and first article inspection are already done. Ask for a price break table with the quote and you will see it directly.
Can I machine a prototype and mould production parts?
Yes, and it is common, but the two processes impose different design rules. A machined part with sharp internal corners and no draft will need reworking before a tool can be cut.