How much should a basic 340 rebuild cost?

I'll tell you one thing as a small shop engine machinist.
Be thankful anyone wants to be a engine machinist.
As an engine machinist I can verify, it is a very underappreciated and underpaid job.
If I had to pay for any of my machine work with my wages I wouldn't be able to afford it.
I don't use the shop enough to call it a benefit either.
Agreed. I'm surprised engine machine shops are still around...perhaps rates have caught up in the last few years but it was pretty competitive. I saw last night that the local low quality neighborhood chain automotive shop wants $155/hr and they probably do most jobs in half the allotted time so you are paying closer to $300/hr. A machine shops tooling is far more expensive than an auto repair shop.