Ch racing doesn't sell Marathons anymore. Actually back in the 90s, they were sold mostly to European privateers that wanted to race the African competitions.
You can still find some in Europe. They are rare. The problem is the fact that most of them raced in several competitions so that they are beaten up pretty bad. It is very hard to find one that was never utilzed in such competitions. When it came out it costed almost 40 millions liras, which translated at that time in 22-25K dollars.
I agree with the fact that the bike is mostly a collectible therefore how worth is it, it all depends on what you want to do with it. I had the opportunity to buy it and I went for it.
I bought the Marathon in France (Alsace) more than a 16 months ago. The bike has only 3K miles, all done on pavement.
How much I spent? 7k euro but I was extremely lucky to get it at that price. I drove there, pick it up and brought it back.