This photo provides before and after views of a part that was machined (top photos) and the same part after it was converted to die castings (bottom photos). The die cast part is able to capture much more three-dimensional detail and consistently meets close tolerances during the manufacturing process.
I agree that converting machined parts to die casting usually makes sense. Because die casting tools can be expensive, it is important to first do a pay-back analysis and see if the volumes justify this change over.
In many cases we use both processes during the life of the product. When the initial design is likely to change and we need to enter the market quickly, we may start with a machined part. Then, as the design becomes stable and production volumes increase, we plan for a smooth cut-over to die cast tooling.
In most cases, casting a part versus machining it from bar stock is a no-brainer. In my career, I've only come across one part that made more sense as a screw-machined part than as a die casting. In that case, the geometry of the part made it extremely easy to screw machine. Also, screw machining allowed the part to be made out of a much stronger wrought alloy. It wound up being an 80% cost savings (from $4 to about 80¢), along with a more than 50% increase in strength.
But this is far from the norm, and as this article shows, casting is almost always much cheaper. A more interesting comparison would be between die casting and powder metallurgy. It would also be worthwhile to compare different casting processes (die casting, semi-solid processing, permanent mold, investment casting, lost foam, etc.). In addition to cost, these processes also vary in terms of the mechanical properties and dimensional accuracy that can be achieved.
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