Separate geometry cost from commercial scope
A high quote can reflect difficult geometry, expensive material, low quantity, urgent scheduling, finishing, or documentation. Ask for the dominant drivers rather than assuming the shop is charging too much. Two quotes may include different inspection and finishing requirements even when both say CNC prototype.
Identify what is one-time setup and what repeats per part. A small quantity can carry a substantial setup share. Ordering more units may reduce nominal unit cost while increasing total cash exposure and obsolete inventory. Compare the quantity needed for the next decision before chasing a lower per-piece figure.
Protolabs' milling guidance describes supplier-specific size, feature, and tolerance constraints. It is useful as a screening reference, but another shop may use different equipment or methods. Let the actual supplier explain its approach to your part.
Review tool access and internal geometry
A rotating cutter naturally leaves a radius in internal corners. A perfectly sharp internal corner in CAD may require a different process or a design change. Ask whether the mating part actually needs that corner; a relief or larger radius may preserve function while simplifying machining.
Deep narrow pockets and long slender tools can increase difficulty. Thin walls can deform or vibrate. Features on several faces may require additional setups or equipment. These relationships explain why removing material from a simple block is not priced solely by the final part's weight.
Do not apply generic ratios blindly. The feasible geometry depends on material, tools, setup, accuracy, and the shop's process. Request a marked-up model showing the specific features the supplier would change, then review each against the functional requirements.
A hypothetical sensor-mount prototype
An inventor designs a small noncritical mounting block with a deep rectangular pocket, sharp internal corners, and six cosmetic scallops. The drawing applies a tight tolerance to every dimension. The quote is higher than expected, but the functional need is only to locate a purchased module and provide two mounting holes.
The shop explains that the pocket corners and depth require difficult tooling, while the blanket tolerance increases inspection effort. The inventor checks the module's actual envelope and finds that larger internal radii fit. The pocket can also be shallower without affecting the mounting relationship. Cosmetic scallops are removed from the prototype scope.
The critical locating surfaces and hole relationship keep their justified requirements. Other features use an agreed general tolerance. The revised quote is requested with the same material, quantity, and finish so the effect of the design changes can be understood. Any savings are project-specific; no universal percentage is promised.
The revised part is then checked for fit and access. If the larger radii obstruct a connector or the shallower pocket changes the module's reference position, the modification is not acceptable despite a lower price. Cost review must remain tied to the assembly.
Question material and finish only where substitution is valid
A different alloy or stock size may simplify purchasing or machining, but material changes can affect stiffness, corrosion, weight, or later test relevance. Ask why the original material was selected before replacing it. If the prototype only tests geometry, a substitute may be appropriate with a documented limit.
Finishing can add handling, masking, lead time, and minimum charges. Determine whether the current experiment needs the final finish. A sliding or sealing interface may depend on surface condition, while a basic envelope study may not. Omitting finish is a scope choice, not evidence that the finished product will behave identically.
Inspection should be proportional to the requirements but not removed indiscriminately. If a critical fit is the purpose of the prototype, eliminating its measurement report to reduce cost can undermine the whole experiment.
Prepare a focused cost-review request
Send the supplier a list of functional features and ask for the top cost drivers, feasible alternatives, and assumptions. Request separate options where useful: current geometry, simplified geometry, or different quantity. Keep each option's definition clear enough to compare.
Update the CAD and drawing together after accepting changes. Retain the original and the reason for each revision. A supplier suggestion in an email does not become an unambiguous manufacturing definition until it is incorporated or explicitly approved in the agreed process.
Your deliverable is a revised quote tied to a revised design, with critical function preserved and tradeoffs documented. That provides a defensible cost reduction rather than a series of arbitrary simplifications that make the prototype cheaper but less informative.