The age is not as important as how well maintained it was over the years. I have an Exmark Viking (hydro drive), 36" width, 15 hp Kohler. I bought it in 1996, and was my main workhorse for 10 years, running about 600-700 hours per season. Now, I use it about half the time, after I added a ZTR three years ago.
I just ran it today -- 9:00 to 7:00, running a good part of that time. It cuts just as well now, as when new. I've changed out the engine one time, and have replaced spindle bearings a few times. Of course, deck and hydro belts have been swapped out too. Yes, the linkages in the controls are pretty loose, but it still makes me money, without any significant repairs. The most money I've spent in the past three years is one deck belt, one hydro belt, and a set of spindle bearings. Engine oil gets changed every 40 hours, filter every other time, hydro fluid change each season with a new filter.
I'm in an environment where the deck does not get damaged (e.g. sand erosion).
I have no idea about price. When I retire this machine, I will have no idea on what to ask. It has made me so much money over 13 seasons, a couple of hundred here or there is trivial. I anticipate I will wear out before this Exmark wears out.