I normally see these for $600-$1200. Sometimes you can find them for less, but usually the finish is in rough shape and the gun may need some work on it. Good luck with the replacement parts. They exist, but they're not cheap. Grips are easy enough to find because Python grips are still readily available.
If you're going to purchase a Colt Trooper or Colt Three Five Seven (same frame, different finish and vintage- the Colt Three Five Seven preceded the Colt Trooper series), my advice is to get one in good shape. You'll only know this if you handle the gun. Outward appearances aren't an indicator of the serviceability of the internal parts, which is what you need to know if you actually want to shoot them.
The Colt Trooper Mk III and Lawmen are different guns internally and parts are not interchangeable. The Mk III is still a good gun, but it does not have Python internals. The frame and barrel of the original trooper are svelte variants of the Python barrel and frame. Sometimes the frames of Troopers were welded up, the barrel was replaced with a Python barrel, and they were advertised as being Pythons, which they were not. Sometimes these guns were referred to as Poopers. I like the balance of the original Trooper and Three Five Seven better than the Python. The extra weight that the Python has makes it more pleasant to shoot.
In the '50's and '60's, Colt made some really nice revolvers, SAA included.
There was a gradual and slight reduction in quality, mostly because the original Colt Trooper and Colt Python were expensive to produce in terms of labor costs. Labor was cheaper in the '50's and '60's. Labor costs have gone way up whereas cost of materials and machining has gone down.
The Mk III, Lawmen, Boa, Anaconda, and some other variants thereof have more cost efficient actions, but they're not quite as smooth as the Python action. They're still good quality revolvers and generally cheaper and with better parts availability.