I'm pretty sure they can sell for a decent amount........? I've seen them for under a $100 and also for a few $100. I'm sure it depends on the year, and need. I found this link, unless that's your posting....... .......? 2004 Cheverolet Silverado/GMC Sierra service manuals | eBay
Check on eBay for what they are going for. That's what I do whenever I find have something that's of no use to me anymore. Hop on eBay and see if any are selling, go to completed listings and see what they have sold for. I've recently come into a lot of old junk from my folks home when they sold it and moved into assisted living. My dad had a old HO train engine and tender in a box that was torn up from years of grand kid abuse. Don't even know if it worked, but listed it on eBay and sold it for $50. Win!
Think pallet pete dropped some good coin for service books, he posted it in his thread not sure what he paid think it was over at least 100 bucks. I have seen 5 book repair books set on ebay go for a few hundred easy if they are in good shape.