As big a risk as they both are, they are being paid at market value. And, like Candy's deal with the Nationals, both have incentives built into their contract as well.
The Tigers were in a tough spot with Candy... unlike all 29 other teams who had the opportunity to negotiate fair value for him after being non-tendered, the only real chance they had to keep him was to pay him the full monty. I can understand both sides, but ultimately baseball is a business and teams (not just the Tigers) aren't necessarily in the business of overpaying players to the tune of 40% or more over their market value.
As fans, a lot of us will not get it because it ain't our money. But it's understandable why they did it and I imagine Harris knew what he was doing and knew the risks when he did it.