I like a little bit of diecast cause I like my toys to have a feeling of heft. I think generally speaking, the lighter feeling it is, the cheaper it feels.

Good design can really make diecast work across the entire figure though. The recent King of Beast Golion is a fantastic example - probably 80% of it is metal but the engineering is top notch so there's no sagging at the connection points at all!

Transformers tend to have more moving parts (and parts that shift place between modes) so it becomes a bit more difficult to place the metal. I'd say they've done a good balancing job with the larger MP toys.