A year ago, you could safely assume $4-$10k on the aftermarket, or $20K + to an end user. Today, not so. Most companies are already online, and if not, they're not ready to spend $10k in a post dot-com era.
$5,000 only because as an acronym, there aren't many professional options for company names that contain an X in it.
Otherwise if it was NC or a domain with endless potential, I'd appraise it at an easy $75,000-$125,000.
I bet you could get alot of money from this domain, probably over $5k. You may want to contact the company who's stock symbol is NX, they might be interested. Also, NX is good because there is no way to trademark the domain.