The condition of the car has a great deal to do with what it is worth. I suggest doing two things. Check autotrader and do the advanced search and input the data for the car which you are thinking of buying as well as the year prededing it and after it. I would also do the same thing looking at eBay.
There is no hard and fast rule on these cars because it varies so much. I would laugh at anyone who offered me $17,900 for my M3 which is a 97 M3/4 because of its condition which is as close to perfect as I will ever get. However you can find the 97-99 year groups selling for that price if you are careful.
An M-3 that has been tracked would never be a car I would own due to the rear sub frame possible failure. Some guys are lucky and don't see the problem and others have found damage due to tracking the car so be sure you know how the car was driven.
A 5 spd manual will be more desired then a 5 spd auto as it is more fun to drive, more bullet proof then the 5 spd auto tranny that BMW bought from GM.
If I was to offer a value I would be looking to pay anywhere from $20K to $16K condition and equipment dependent. This assumes that the inspections and services have been preformed and the car is in great shape.
My data comes from owning an M-3 as well as still have a SC 325iC and having owned 4 other e36 models and doing all my own work on these cars because the dealer in my area is not capable to doing quality work.
What to watch for is what you would watch for on any 6 cylinder e36, water pump, radiator, t-stat housing, control arm bushings, shocks and struts which are all pretty well shot by 70-90K. How these items were replaced if it has that mileage and what parts were used (if that is possible to determine) give you an idea of what you would have to do to keep the car 100% down the road.
Frankly these cars are not that far from bottoming out. Even though there are lots of e36 M-3's out there they will be getting close to the bottom and will most likely maintain their value as long as they are maintained.
Hope this is of some value...