The .22 Magnum Trooper Mark III was a fairly low production model.
Value depends entirely on actual value, but a good starting figure would be at least $350 or more.
The Mark III is certainly the strongest .22 rim fire revolver ever made, and the guns are tanks.
Accuracy is almost always astounding and the blue job is a quality no longer seen.
What to look for is the usual "dinked up" screws, whether the grips are original Colt walnut Target types, rust, pitting, any signs of abuse, corroded bore or chambers, battered cylinder locking notches, etc.
The Mark III should be dry fired ONLY with empty cases in the chambers. If dry fired empty, the firing pin "may" break, and this requires a trip back to Colt to have a new pin installed. This IS NOT a do-it-yourself job or a job for the local gunsmith. It takes some special equipment.