By drivetrain, specifically the chain, cassette, bottom bracket, and crankset on a hybrid bike with Acera/Alivio level components.
One shop quoted $320, one $170, and a shade tree type guy $120. I went with the middle one, just curious what something
fair/typical should be.

You can look up the cost of the parts. It's likely the labor that will vary.

Here's the deal: Giant, Trek, Specialized and the rest all pay lower prices for bicycle components than you do. Replacing a lot of components on a bike is generally a recipe for turning a cheap old bike into an expensive old bike that has a few new parts. The cheap solution is to find a bike that's broken component and grab all of the unbroken stuff off if it. That's probably how your shade tree guy must get his components if he's willing to do that much work for only $120.00.

What are you looking to accomplish? Replacing components with similar, higher quality components will probably not result in a significantly better ride. I recently replaced the drive train on a hybrid specifically because I wanted to go from a 3x8 to a 1x10 with a different gearing range. I also wanted to go with wider rims for wider tires. I bought NOS when possible, taking advantage of either take-offs or discontinued and was able to keep my price less than $150 for crank, hubs, rims, cassette, chain, and RD. All work was DIY so probably saved a bundle there.

For a fair comparison you'd need for the shops to provide a list in which all the new components are the same ones from shop to shop. Variation in parts would account for some variation in the price with labor the rest, but labor rates can vary widely from one shop to the next. That's a big range you quoted but without more detail there's no way to know what's fair. You also have to consider the "get lost surcharge" on some jobs; if a shop is slammed they might not want that particular job and might charge you accordingly if you insist on proceeding.