The DPR is roughly $3000 with 2 gaskets all bolts and maybe a temp sensor, they should also update your ecm software and adjust your exhaust brake after the repair is performed. It sounds like the cam housing is leaking as well,. Warranty pays 5.3 hours for a cam-housing reseal plus parts. I charge 8 hours and that includes a valve adjustment. If you are just doing injectors we charge the customer 5 hours plus parts. So 6 injectors, 6 fuel supply lines, 6 o-rings 8 return line washers, 1 fuel return line, 1 valve cover gasket, 1 fuel return "x"bolt , and 1 fuel filter comes to roughly $4600 in parts give or take. These are old prices. If you are re-sealing the cam-housing add some misc , a cam housing gasket, 7 rocker-shaft bolts and some anaerobic to the parts quote. It will not allow me to pull up past info from the portal. Leaking fuel injectors could cause and is in fact a leading cause of DPR filter failure. If you could get a print out of the balance rates when the truck is up to operating temp and a list of codes I could possibly help further. It sounds like they are giving you a accurate diagnosis. It can be hard to swallow a 6000 repair bill though I'm sure.