The timing belt service that most other maintenance is based around is listed at 100,000 miles for the non-turbos however many bump that up to the Twin Turbo listing at 60,000 due to relative age of the belt. Which is an important consideration as well, even it was done 30,000 miles ago I would probably replace a belt that is 7 years old. A full timing belt kit with timing belt, coolant hoses, water pump, gaskets and etc is around $350. However I would base maintenance expectations on the individual car. They've been out long enough that there is no general mileage or maintenance condition across the 300ZX stock of America so if you can find a particular car with x mileage or y amount of known maintenance we can probably price it out here for you. The biggest concern is whether or not you are going to be doing the work yourself or if you are paying for labor. Most Z maintenance is not difficult as wrenching goes, just time consuming; with the packed engine bay you have to move or remove a variety of things to gain access to other things.
Most NA owners get in the low 20s from what I have read and people I have talked with.
300ZXs are pretty reliable, there isn't any parts they eat regularly or problems that crop up in every Z over time. The early model (90-94) injectors don't like ethanol blended gas that is available some places and have started dying with prolonged exposure. Wiring harness on early years tend to be brittle and sometimes have nasty cases of corrosion in the connectors that has to be cleaned out. They surely aren't a "What is maintenance?" stripper Corolla but if the car was taken care of it shouldn't suffer persistent problems. Parts can expensive as far as imports go but it's not too bad.
I looked up the KBB for a mid-year 300ZX non-turbo in KBB "Good" condition with around 100,000 and they reported around $6000 from a private party, which is fair price if it is in sound mechanical condition and clean overall. I also checked Zs for sale in the 78130 and the pickings are kind of slim (closest one shows as 36 miles away) but most of the ones I saw on autotrader seemed like they should be fairly clean cars. And most of them fell within an acceptable range of the KBB value and my personal evaluation on price.
Here is the search I ran.