First question, what part of ND are you in?
Second part - it will depend on what options it has? 18x10 rear wheel upgrade or just 18x9 all around? How are the leather seats? Is it one of the 'Ralley' editions or does it have side exit exhaust? Which mufflers (short or long)? Which brakes 13" all around or 14" front, 13" rears?
I've seen super low mile 'rare' cars pull over $20K if they are in good shape. The real factor becomes the fact that most of the good condition Stage 3 cars are being held onto tightly, so finding one for sale is hard, and you end up paying a premium.
And just b/c I'm curious, what color and what serial number?
1998 Roush: M112'd Aluminum block 4.6L 2V 6 speed track car
1999 SVT: Mostly stockish
1970 Mach 1: Resto-modded monster
2000 Roush #5004 - World's FIRST M112'd V6 Mustang. Sold, but not forgotten.