the value of anything is the product of many factors one of which would be local availability and desirability of any given thing.
Any Weltklang alto or tenor, older or newer, won’r excite anybody over here in the NL and would sometimes sell for less than 250€.
If you can buy one of these older silver plated “ Solist” model (or the B&S so called blue shield which are contemporary and made by the same factory) and fix them yourself then you could, perhaps, sell it at a profit on a market where these horns are valued more than the market where you’ve bought them.
Is this your case in Norway?
I know I couldn’t buy it in the Netherlands, have it fixed (I don’t do this myself) and re sell it at a profit. Even the cheapest of the technicians would ask 300€ for an overhaul these days which would automatically bring this sort of horns in an unsellable range (unsellable because people would buy something else with the same amount of money).
This is a different situation in case you overhaul the horn yourself. But in reality all you do is selling your overhaul, that’s where the profit lays.
Unfortunately this state of affairs is affecting all manners of saxophones in the lower priced segment.