Grad students are actually more expensive, because they're slower and more likely to mess up than a pro would. However, you still want the grad students to do it.

A successful experimental physicist needs to have a deep feeling for how their experimental apparatuses work, including all the grungy real-world details that theoretical models don't cover.

You get that feeling by building and using the tools, and since the point of being a grad student is to learn how to be a successful experimental physicist, they've got to be the ones who build the tools!