I. Avoid creating designs with sharp internal corners and use inner radii at the corners of your design.
II. Where possible, design all parts with standard sizes. For instance, it is more affordable and cost-effective to make parts that measure 10m in height than 10.5m, as the latter requires human intervention.
III. Try to design parts for CNC prototyping without a draft.
IV. Simplify part design to reduce the angles and sides from machining. With more angles, axes, and sides for part machining, the cost would be higher.

The cost of any CNC machining project depends on the type of material and its cost, machining time, tolerances required, need of hole tapping, and finish.