Hard to know for sure. My 2015 GMC 2500 was under GM's free 2 year maintenance and when I did the 4 free oil changes they topped off the DEF tank for free also. I think I've purchase 6 2.5gal bottles at Walmart. Total miles to date is 29,830 and the DEF was topped off just before I got home. If I had to guess, it probably uses about a gallon every 800-1000 miles. I buy the $8.87 Walmart brand or the Pilot J bulk ($2.90 gal.) and that works out to about 1/3 to 1/2 cent a mile.
2016 Wildcat 295RSX - 2015 GMC 2500HD DblCab Duramax/Allison 4x4 Z71 6.6' Bed
Maxxis 16" E; Ride Rite AirBags w/AirLift 1 wireless compr; Adv.Folding 4 Cover
Andersen Ultimate II Aluminum 5erhitch on Reese Flipball w/Curt 4" offset ball
LCI Ground Control 3.0 4pt + 2pt Elec.Rear stabilizers; 3 Slidecovers; 10ft. Portabote
6000# Dexters; mor/Ryde CRE3000 w/HD Wetbolts; King Dish Tailgater; 5hp Outboard
Dual A/C; Furrion 48 Rear Camera; Champion 3100 Inverter w/Extended Run