Cheapest and most efficient are different strategies.
I probably did the cheapest method when I was prototyping my battery holders a couple of years ago, pattern in card-backed foam, and then a silicon mould.
This produced a passable trial part, but it wasn’t very pretty!
3D printing quality is fantastic, and the new pattern produced great results.
Depends what resources you have available freely, and of course the type of part you are designing.
Cheers, Rich