With a car of that age and miles, there are too many variables for any "rule of thumb" valuation.
You need a checklist that includes everything expensive and everything commonplace.
Injection pump and nozzles
turbo function (the turbos themselves are very reliable, but the vacuum lines can be problematic)
record of last timing belt change
alternator, starter, AC unit, power steering
brakes pads and rotors, shocks and struts, tires.
exterior stuff likes doors, windows, locks, and of course rust or crooked body panels
If everything works, nothing needs immediate replacement, then you can afford to pay a little more. If there is a lot of work needed right away, then lower your offer accordingly.
A 1999 is the A3 or Mk3 model, the one preceding the 1999.5-2005 Mk4 models. These can be great cars, but they did not have the same high level of corrosion resistance as the newer model. So check for rust.
