Machining metal is subtractive: you buy a solid piece and pay to remove everything that is not the part. That makes two things dominate the price, how much material has to come off and how hard it is to cut, and it means the cheapest machined part is usually the one that started closest to its finished shape.
The processes and what each is for
Turning for anything with an axis of rotation. Milling for prismatic shapes and features on faces. Grinding for surfaces and tolerances beyond what a cutting tool holds. Electrical discharge machining for hardened material, sharp internal corners and shapes no cutter can reach. Most parts use one or two of these; a part that needs all four is telling you something about its design.
How the alloy changes the bill
Aluminium cuts fast and cheap. Free machining brass cuts faster still. Mild steel is straightforward. Stainless work hardens and demands rigidity and correct feeds, so it costs appreciably more per unit of material removed. Tool steels, titanium and nickel superalloys are slower again and eat tooling. The material line on a quote is often the smaller half of what the material choice costs you.
Where the money actually goes
Setups, because each one is programming, fixturing and a first article. Material removal volume. Tolerances, since each tight one is a slower pass and an inspection. Surface finish callouts. And features that force small tools into deep cavities, which is the single most common avoidable cost in a machined part.
Buying it well
Send a model and a drawing that distinguishes critical from stock dimensions. Ask for a price at your quantity and at ten times it. Ask which feature is driving the price, because a good shop will tell you and it is often one radius or one tolerance. And let the shop pick the stock size, because they know what is on the rack.
Questions people ask about metal parts machining
Which metal is cheapest to machine?
Aluminium and free machining brass, by a wide margin. Stainless, tool steels, titanium and nickel alloys cost progressively more per unit of material removed.
Is machining or casting cheaper?
Casting wins in volume because it starts near net shape. Machining wins at low volume and where tolerances or material certification make casting awkward.
How do I reduce a machining quote?
Open internal corner radii, reduce the number of faces that need features, loosen tolerances that do no work, and increase the order quantity so setup spreads.
Do I need a drawing as well as a model?
As soon as any dimension has to be right, yes. The model gives the shape; the drawing says which dimensions the shop is accountable for.