Short answer: They are good tractors, but it would have to be extremely nice to be worth more than $500.
The 1981 model should have a serial number plate on the console facing the driver's seat between 155,001-190,000. The model number will be a C216 plus another letter. The 16 hp engine is comparatively uncommon compared to the smaller engines. The tractor should have a manual PTO clutch. Parts could have been changed our over the years. Like El Toro posted, it's 32 years old. The 200-series is not comfortable for someone who is really tall or really big around.