I was quoted $5K + tax for the injectors on my 01. Spent some time on the diesel forums and bought a set from LDS for $1700 and did it myself. Probably cost $2,500 with all the parts, fluid, taxes etc. so I saved around $3K but it took 2 weekends to do (book is 16 hours) and was a royal PITA. Nothing difficult, just annoying. Especially the one valve cover bolt at the firewall on the drivers side. Probably spent an hour in total on that one getting it undone and tightened again. Could not see doing it again.
It's just the first version of the Duramax (01 - 04.5) that has the issue with replacing them. You have to pull half the top end off to get at them. Second generation let you pull them through the valve covers. It was a dumb design.