2 years ago my dad traded in his 2003 F350 CCLB 4x4 auto Lariat with 80K miles. The Ford dealership gave him $22K for it. It sold 2 days later for $25K. It was in immaculate condition, all they did was check the fluids, didnt even wash it or vacuum it out. I know this cause i have a few buddies that work at the dealer and listed the truck for sale personally. Not a single scratch on it. This is up here in WA where i think the market is probably at its top prices fro these rigs, at least in the Seattle area.
Now they probably gave him a bit more cause he traded it in for the same truck but a 2011, and he is loyal to that place, his 4th diesel from this dealer but at the same time the customer bought it for $25K.
I paid 13K for my 2000 F-250 xlt XCLB 6 speed 4x4 with 150K on it. Was in great shape with service records from day 1 with every receipt for every dollar ever spent on the rig.
I think you could easily get $20K, if you take your time you might find the right buyer that is willing to spend up to $25K if you play the game right. Like said above, these truck are in high demand and people are willing to shell out the cash for a low mile good shape truck.
Good luck on the sale!