Honestly - you had the conversations and i didn't, but if they really said they don't want to dig through rock, i'd probably run away.
I've got a lot of experience in this, and what you are being given sounds like a bunch of nonsense, and yes sometimes you will get a bunch of nonsense from more than 1 HVAC firm.
Lots do not specialize in geothermal, and see it as a way they can make additional profit off some people.
Concretely: At the price you are being given, the drill price should be fixed and included, excepting them hitting lots of water.
The driller certainly gave a fixed price, based on the local geography and what they are gonna hit. The know exactly what they will hit in terms of rock/etc.
The variable cost thing is not drilling rock or whatever, it's controlling water and sediment.
If they hit lots of water (IE 100+gallons a minute), controlling and remove it can be expensive and unexpected - most cities/etc have strict rules on this (so they can't just run it into the street). This is the thing that is hard to tell for them - it's rare to have good enough data to know exactly whether and where you will hit water, and it can even vary from hole to hole in the same job.
Otherwise, be careful - unlike regular HVAC, Geothermal requires people who really know what they are doing, and not just doing the kind of "finger in the wind" HVAC you can often find for folks doing heat pumps/furnaces. At a minimum, make sure you are using IGHSPA/etc certified folks.
If you want to get more into this, there are lots of knowledgable drillers and installers over in places like the geoexchange forums, and they won't steer you wrong.
As for Bosch, the units they are offering in residential are super basic units last i looked.
They are neither variable-speed pump or variable speed compressor.
They are simply 1-2 stage geothermal units. They appear to at least offer variable speed blowers on some of them, but that's very basic stuff.
This is the kind of unit you would have seen 20 years ago.
I would not pay what you are paying for them.
Nice geothermal units (say, the waterfurnace 7-series) these days have variable speed pumps and compressors.
They match themselves exactly to the load needed by the house over time, and run continuously at part-load, modulating up and down as they day goes on, and end up being both silent and very high efficiency.
They are also much longer life - parts like scroll compressors (and most forms of rotary compressors) are much happier if you modulate load rather than start/stop.