Not a lot of LL.be reported sales but two stand out as reference points to me:
XR.be just sold for $249 at Sedo. Not a good look - XR is a pretty big term and that's a tiny amount.
Ga.be sold several years ago for close to mid $xxxx. My hunch is the buyer bought it as a hack of "Gabe" rather than a two letter .be.
So what's "un" worth. Are there companies are there in Belgium that may use UN as an acronym? If so, one of them may want it, and depending on how badly they want it, you could get $5k or $50k for it. But if you've had it for 20 years, I imagine one of these companies would have reached out to you by now with interest or offers. So you be the judge there.
On the wholesale market right now, I don't think you can get more than a few hundred bucks for it.