http://www.mlbtraderumors.com/2013/08/2014-free-agent-power-rankings-2.html
Garza rated in at #4 overall on mlbtrade rumors last season. That didn't lead to a lucrative deal for him. I'm not going to pretend to understand FA spending but what I do know is there are haves and have nots in FA. Garza appeared to be a have prior to the signing period and after was clearly a have not. Whether or not Shields will be a have or not is debatable. I'd argue he's a better pitcher than Garza but if interest isn't high in him the same thing that happened to Garza will happen to him.
Since 2006 these pitchers have gotten more than 4 year deals
Tanaka(age 25 7 years), Greinke(age 29 6 years), Sanchez(age 29 5 years), Ryu(age 26 6 years), Darvish(age 25 6 years), C.J. Wilson(age 31 5 years), Cliff Lee(age 32 5 years), Lackey(age 31 5 years), Sabathia(age 28 7 years), Burnett(age 32 5 years), Zito(age 29 7 years), Gil Meche(age 28 5 years), and Kei Igawa(age 27 5 years).
If we use that as a guide, you're typically talking about one big guy(Scherzer) at 6-7 years and potentially second fiddle guy at 5 years. And based off this info, you're not seeing many guys over 31 signing for 5+ years with Lee and Burnett being the two exceptions. Perhaps you can argue Shields is as good as Burnett was. He had a 4 ERA the year prior but I frankly think it was more the yankees being the yankees there and bidding against themselves. Lee doesn't seem like a good comparison.
Overall, I could see him getting a deal like Mark Buehrle got(4 years $58 mil) though obviously increased based on inflation. Dempsters 2008 deal might also be a decent comp(4 years $52 mil). Something in the 4 year $60 mil range seems appropriate unless two teams go crazy against each other. Perhaps you throw in some sort of vesting option for a 5th year but I doubt he flat out gets it. The vesting option is how the Brew crew handled Garza. It's a 4 year $50 mil deal with a 5th year vesting option for $13 mil or a $5 mil team option if that doesn't hit.