This will be posted in two parts because of the word limit:
You haven't said how long the boat is, its build quality, where you are living or what condition. These are the main factors in determining the value of a steel boat. Steel boats vary in fair market value far more widely than glass boats. Partially because glass boats are comparatively closer to a commodity item, where as steel boats with any age on them, are pretty much one of a kind.
Steel boats that were built in Europe during the 1970ís tended to be normal steel. typically red lead oxide primed and painted on the interior. 30-year-old steel boats built that way have very little value unless they have been replated and coated with a more modern sealing system. There is no good way to recoat one of these old boats because you canít get behind the framing where the rust is typically located. I would expect a boat like this to have very little fair market value compared to a similar initial quality, similarly maintained, similar design glass boat of that era (perhaps 20-30%of the value of the glass boat). I would also expect it to need extensive work perhaps equaling several times the fair market value of the boat, unless you are able to do the work yourself within a short period of time.
The following is part of a series of articles that I had written for another venue, but which might prove helpful to you:
EVALUATING OLDER STEEL YACHTS
It is never easy to set a fair market price for an older limited production boat. This makes evaluating a fair market value for most steel boats especially difficult since most were built on a one off or semi-custom basis. Adding to the problem is the bad reputation that steel has gotten from poorly constructed home built boats. If I had to work my way through determining a value for a older steel yacht, I would rate the boat each of 5 categories; 1) Original Build quality, 2) Finish, 3) Design obsolescence, 4) Maintenance, and 5) Location, and then sort of multiply the ratings out.
Original build quality:
Steel boats vary very widely in price because the boats themselves began life varying very widely in build quality, in terms of materials used, building technique, and quality of construction. There are wild differences behavior of the various steels and coating systems used and consequently in fair market value. The variations can impact on the strength and rust resistance of steel. On an older boat, mild steel pretty much reduces the value of the boat to its salvage value whereas if the boat was built using some of the more exotic steels and coatings, the boat can be worth as much as, or even slightly more than a similar sized glass boat.
Whether the boat is round chine and fair vs. hard chine also compromises it value. Independent of all other rating factors, a radiused hull steel boat can worth as much as a glass boat of equal age and size. A hard chine or folded hull can be worth as little as half as much mostly because of perceived aesthetic and sailing ability issues.
There are also wild differences in value based on the build quality. By this I am not talking about the finishes but more about the quality of the welds and connections, fairness, and other aspects of workmanship. I have been in the bilges of steel boats that were work of art and steel boats that were simply scary. A well built steel boat adds nothing to the score but a poorly built one reduces the boat to the negative cost to cut it up and haul it to the junk yard because in effect you will eventually need to build a new boat that is essentially a replica of the old boat.
This is a very subjective area. To begin with, just like most boat building materials steel boats have been built with beautifully finished yacht levels of finish, with purposeful workboat levels of finish, and with cobbled homebuilt levels of finish. A good case can be made for each of the first two, but the third should be walked away from because a truly crudely built steel boat can have life threatening problems such as bad wiring resulting in sudden massive electrolysis.
In my life I have been aboard 1950-60ís era Dutch build steel boats that were extremely elegantly finished with levels of fit and finish that were fully on a par with and perhaps even nicer than Hinckleys of that same era. But steel boat interiors (or wooden boats for that matter) have a special challenge. On a steel boat it is important to be able to get to the interior of the hull periodically and so no matter how beautifully it is finished it needs to be removable for long-term maintenance. On an older boat, if the interior is not removable, a yacht level interior can be a liability and not an asset, because not only do you have to remove and dispose of the existing interior but also on a yacht finished boat, the next buyer expects a replacement with similar yacht level of finish to the original.
Yacht finished steel boats are substantially more expensive to maintain than either fiberglass or workboat finish level boats. In a marine industry study of life costs as they pertain to various materials, steel was the most expensive material to maintain over long period of time (even more expensive that wood for example). Some of this reflects the need to replate and possibly reframe at some point, but it may also be skewed by the sheer cost to maintain a yacht finished steel yacht, which require being periodically refaired and refinished with high tech finishes to maintain that yacht finish.
As a result many steel yachts purposely receive workboat finishes. There is nothing inherently wrong with a workboat finish. They are inexpensive to build and inexpensive to maintain. Workboat finish does not mean poorly built, with cheesy levels of fit and finish. Instead, workboat finishes are simply detailed, low gloss, using solid but basic materials. Maintenance is much easier because a small repair does not result in a major repainting and refairing job, and their low gloss finishes can mask a whole range of minor defects.
But while a workboat finish is perfectly respectable and offers real advantages for distance cruising, they are generally of less inherent resale value than similar sized, aged and condition, yacht finished fiberglass or steel boats. They typically sell for prices 1/3 to 50% less than the value of a yacht level boat.