Over paying for a closer is about the dumbest thing you can do. You can get solid relief for relatively cheap, better to spend the money on starting pitching.
I'm not worried about the closer spot. Even though Marmol hasn't been consistent, 1. He's got the stuff, I could stand to see him get another couple years to work it out, and 2. Closer is one of the LAST priorities on a team with needs like the Cubs have. A closer can serve as the final piece of the puzzle (see Brad Lidge for Philly in '08), but unless your team can GET to the closer with the lead, even Mariano wouldn't have much value.
You don't build a team for the regular season. You build a team for the playoffs. Closer is arguably the most important piece of the puzzle. How many teams win the World Series without an excellent closer? Last year, Brian Wilson was awesome. Rivera, Papelbon, Lidge, Soriano, K-Rod, etc. all come to mind.No kidding. Spending money on a closer is incredibly wasteful. I bet Carpenter could be a solid closer for us next year and he won't cost more than the minimum.
Now, I won't say that's a bigger need than starting pitching, but if you have 3-4 solid starters, getting to the playoffs shouldn't be an issue. Winning those games however is. Your starters are going to give you 6-7 IP in a playoff game, then someone has to take over. Who do the Cubs have for that? Marmol isn't the answer.