Kbb should still be available. Based on what I've seen a minty fresh 525 with medium miles can fetch 3.5k if finding the right buyer. You start getting into the fair cars that need a little tlc have very minor defects go somewhere in the upper to mid 2ks. Real nice cars that have been hit, real nice cars in need of something major, cars that are alright, go between 1.5k to low 2ks. Anything less is in need of help. 500 and below don't expect to keep lol. Hope this helps a little.
2003 GMC Sierra SLT 2500HD CC/SB