According to Phisherman :
Number 6 can run you $500-1000.

It could, but unlikely. It's only 40A. A simple swapping out of
a 4 or 6 slot panel shouldn't cost anywhere near that much.

As another point, a missing ground will _not_ cause a GFCI to fail
to trip. It was either miswired (feeding load not line side), or
the GFCI is defective. GFCIs do NOT need grounds to operate.

It's hard to tell whether $2300 is high or low - many of the things
could take considerable time to fully rectify to current code, which
we can't tell without seeing them.

Eg: how _much_ lampwire has to be replaced? What difficulty is
there going to be in routing it to code, etc? How much trouble
is there going to be in establishing grounds? Can you just
slap a box on the range splice, or do you not have enough slack
and have to run more cable? Is it even the right wire?

It looks like the electrician guesstimated high to cover their
butts for gotchas they didn't have an opportunity to look for,
making an assumption that working in a garage will be easier
than in the finished part of a house.

But I could certainly see it ending being too low, depending
on what "holy crap, lookatthat!"'s they uncover.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.