The first question to ask is at the design phase. Why do you need metal? It might be for structural reasons. For instance, you are building an oddly-shaped, high-torque gearbox that will transmit high loads to the housing of the gearbox, and plastic just won’t do). It might be a measured decision to minimize plastic parts. For instance, you may elect to use sheet metal instead of plastic for internal structures. Each part will cost more, but you will delay the tooling cost. It is a very good strategy if you are test-marketing a first design and are quite sure you will have design changes before scaling up to mass production.
Let’s say you have very good reasons to need metal parts. Sheet metal is a good choice – it is typically a fast turn process with relatively low investment in tooling. If you must create a complex 3D shape, you will probably start with a CNC part. This part will cost hundreds of dollars. The long term goal might be a cast part that is subsequently post-machined to achieve the tolerance needed for that part. But tooling for die casting aluminum is astronomical, and the lead time is usually 4-5 months. In this case, you might have a heuristic for whether to invest in die cast tooling. Let’s say you are making 500 copies of this product. Can you make back the per-part cost in 500 parts? Given the high cost of CNC machining, the answer is often yes. If you can come up with a clear ROI that you can count on, and you can wait 5 months, go for it. If not, you will just have to grin and bear the CNC cost. Although my question to you is: Why on earth did you design a complex metal part? Can you change the design so it can use plastic parts? Sometimes you need to think out of the box during the architecture phase and consider the consequences of certain design choices all the way to the end of the process. What is fast and easy now may create a permanent headache once you go into production.