Which Is Better Alder or Ash?

Sonically, Alder will produce more attack and presence with its emphasised upper-mids, resulting in it sounding a little bit punchier than Ash. Ash will give you a comparably rounder and slightly mellower tone with chiming highs that aren't too bright.

Is alder more expensive than ash?

The big difference is that ash is so much lighter thatn alder. That's what makes it favorable for a guitar body. It is a tad bitter more expensive than alder, that's why you never see it on low end guitars.

When did Fender switch from ash to alder?

Fender adopted alder for electric instrument bodies in mid-1956, probably for no other reason than it was there; it was readily available and more affordable than ash. Ever since, it remains the body wood for the majority of Fender electric instruments. It was and still is a very good choice.