Prices are rising but they're not high.....yet.
It needs a decent clean, new tyres and possibly "recommissioning" (some seals are likely to need replacing - oil/water pump seal will probably fail shortly after it's run for any length of time and the output-shaft O-ring is likely to go south too).
Possibly the best thing about that mileage is that the tyres are original and therefore no-one has needed to remove the wheels and cock it up yet. Removing the wheels without splitting the front wheel fairing seems to be beyond most ham-fisted owners/tyre fitters, then they think that the wheels will come out without major dismantling of the brakes (even with a ramp that has a wheel-drop panel).
Values are subjective. Interest is growing, their rarity is both an asset and a drawback. They are (wrongly) believed to be a complex bike. They were in comparison with what was on offer twenty years ago but by comparison with today's bikes they are simple enough.
Most parts are still available new, only a couple of K1 specific parts are NLA. Unfortunately they are the most fragile of the inner fairing panels (they weren't nicknamed "yoghurt pots" in Germany for no reason).
The biggest problem is that you really need to ride one to understand and few people have done so due to their rarity. That means that the market is reduced to those who wanted one when they were a current model, those who had one once before and want one again or a collector.
There weren't many who wanted one when they could be had new, BMW seriously mistook the appeal of such a single-purpose bike.
Of those who had one twenty years ago the problem is that twenty years later they possibly cannot fold themselves into the position needed to fit in it (and it is in not on).
I've had lots of interest in mine, more than there was when I had one twenty years ago. Whether that would translate to enough interest to buy it is another matter (not that mine is for sale, I made that mistake once before and regretted it bitterly).
Keep it. Go through it, clean it and ride it (if you can). It will never lose money but it will be a few years before it really fetches what it's worth.
