Jax speaks the truth on this one.
The Reds can't pay big contracts at every position. If Sean Casey had taken the Jason Giambi career path (e.g. become a .420 OB, .550 SLG shoo-in), then he might have warranted a long-term investment. Obviously he'd have cost more, but you'd have a player whose name you could ink in the #4 hole and consider the job done for the next five years.
The problem is partially one of position. Smaller market teams need to be smart about where they find their savings. It's a lot easier to find a productive 1B, LF or RF than other positions. If Dunn and Kearns turn into the monsters we expect them to be, that means the Reds in a few years will have to cull some savings out of the IF. Yet Sean Casey is a 1B and he's not an outstanding 1B. Don't get me wrong, he's good. He can help a team win games. But he's not a player you build a team around (and he essentially let us know that during the 2001 season).
Now I didn't mind the Reds signing Casey this year. Reason is because it's a contract other, wealthier teams can afford. It's even a contract that smaller market clubs can handle if they've been good about their budgets.
By my count, the Braves, Expos, Marlins, Pirates, Dodgers, Giants, Red Sox, Devil Rays, Orioles and A's could be in the market for a 1B. That's not to say they'll want Casey or that they'll want to spend Casey money on one, but I imagine all those teams will review that position with most making changes. You might have to trade for someone else's disappointment in the offing, but I'm relatively sure that a market for Sean Casey does exist (heck, there was a taker for Dmitri Young).
The Reds have within their ranks competent (heck, superior) replacements for Casey. They've got to be smart about money. It's getting difficult to argue that Sean Casey is a wise expenditure of the team's money.