My 97 has 89K on it. It stays in a heated garage all winter so it's basically pristine. Great truck and I too have people hounding me for it, dealers included.
Last summer I had it at the dealer that I bought it new from (for a recall upgrade on the cruise control kickoff) and it cleared the service bay out. People had to come out and look it over.
No rust anywhere. I've been offered 20 for mine (4x4, long bed 4 door XLT). Not for sale. OBS 7.3's are in high demand and are getting more in demand every year as people realize the new emissions compliant stuff is a PITA. I have no emissions hardware on mine at all.
One thing about a Navistar 7.3 that is an issue is injector 'stiction'. I run 'Archoil' nano borate oil additive in mine, every oil change. Really makes the engine purr, starts easy and runs quietly. 5K oil changes and yearly transmission fluid changes (plus filter)
Not a hot rod but reliable as a rock. I bought it new btw in 1997.
Is it worth 20? maybe, but it would have to be in pristine condition mechanically and body wise.
Mine was 33 new, fully loaded (XLT). Still have the window sticker in the glove box.