To get an idea of the value of Bandit 250 chipper, look at the asking price of similar machines.
Engine type and size are a big factors in determining the price for the same model chipper.
A diesel engine should be more expensive than gas, and the greater the horsepower rating, the more costly.
Compare options on individual chippers; does one chipper have a feed wheel lift cylinder, for example. The older the machine, the less options and/or upgrades it may have.
The overall condition of the chippers used for comparison, while extremely important, is difficult to judge by photos alone.
A good indication of wear and tear on any machine would be the hours used. However you cannot always rely on the reading on the hour meter, it could have been disconnected or stopped working at any point.
1200 hours sounds like an extremely small amount of usage for a 15 year old chipper.
4 hours per day @ 5 days per week for 50 weeks is already 1000 hours in only 1 year.
Try looking on these sites for comparison:
http://www.machinerytrader.com/
then click on Forestry, then choose manufacturer (in this case Bandit)
http://www.equipmenttraderonline.com/adsearch.html
then choose make and enter model.
Do a search on Google for "Brush Bandit 250" and many hits will be returned with dealers and private sellers.