I've just been messing around in this area on my '87
300TD.

Minimum rotor thickness is 19.4mm, as discussed above.
This is the *width* of the rotor measured where the
pads have worn it down.

It's important to note that 19.4mm is the minimum
thickness the rotor must have *after* the new set of
pads is worn out. Hence it must be thicker than 19.4mm
when the new pads are installed - but by how much I
don't know. The service manual may specify the thickness
at which rotor replacement is required during a pad
change.

I think the thickness of a new set of rotors is about
22mm. Hence if you have a 1mm lip on either side of the
old rotors, you're sitting at about 20mm thickness vs.
19.4mm as the minimum. I think you're on the borderline
of needing new rotors. Given that the brakes are the
primary safety system in a car, I'd sure replace my
rotors. (Did, in fact.)

BTW, the price sounds absurd. I purchased new ATE rotors
from my favorite vendor for $28/each. Front pads were
another $28 or $29.