I do think the injury will bring down his price, especially when you factor in age as well. It may make him fit the bargains that we typically look for in free agency that come Buyer beware. I think Dallas should offer him (and others) incentive-laden contracts instead of the typical cheap one-year deals they like to throw at FAs ... something like two years, $6 million fully guaranteed with a $2 million bonus per year for 80-plus catches, a $2 million bonus for 1,000 yards, a $2 million bonus for 8-plus TDs. That would mean the Cowboys could end up paying him $12 million per year, but if he hit those marks it would be worth it. (I'd even throw in $1 million Pro Bowl and All-Pro bonuses.)
At the least Dallas pays $3 million per year if Diggs struggles. Of course, I'm no cap expert, so I don't know if that would work.