Gas or diesel? wide front? Or less common narrow front? Everything working as it should? What shape is the rubber in?
They sell from $2000/$2500 for poor condition but running, to as much as $4500/$5000 for above average condition. Last one I bought and re-sold, I paid $3200, spent about $600 to get a few things in better working order, then sold for $4950. It was a gas, wide front, good paint, almost new rubber, low hours, and in great over-all condition.
The 50 is a bit of an anomily. In the case of most tractors, a diesel will be of higher value than a gasser. Not neccisarily so with the MF50. The Continental gas engine is a jewel. A diesel that age can be a money pit, so, the gassers sell easier in most cases. We're talking condition here though. A GOOD gas OR diesel would bring good money. A worn out EITHER version, not so good.
Narrow front's aren't that common, but I hesitate to use the term "rare". I save that for uncommon AND valuable because of being uncommon. The most valuable variant of the 50 is the standard chassis. (24" or 28" rear wheels/wide front) The "HIGH ARCH", as the 38" rear wheeled chassis was called, are less common, but not in a way that makes them more valuable. To get MORE, you'd have to find that special buyer who just HAS to have a narrow front tractor.