James Shields is 33 years old this year.
There's basically no precedent for a 33 year old pitcher getting something around 5/$100 unless you go back to Mike Hampton. Can anyone think of the biggest contracts given to pitchers aged 33 or older in their first year of the new deal?
Cliff Lee - 5/$120 for his age 32-36 seasons (vesting option for age 37)
Adam Wainwright - 5/$97.5 for his age 32-36 seasons.
Cliff had three 6.5+ fWAR seasons in a row before his deal.
Waino went 5.3, 5.6, (missed year), 4.1, 6.3 before his deal kicked in.
Shields has never had a 5 WAR season, but in an 8.5 year career he's had 7 years between 3.5 and 4.5 fWAR. He's also the most durable looking SP in the league right now, more or less - pitched the most innings in baseball since 2007 (regular season) and the only SP with more starts is Verlander, who is kind of broken. The durability is a plus but the "wear and tear" is a negative.
If he's a durable, 33 year old SP that is very good but decidedly not great, then what he is, essentially, is a sexier version of Mark Buehrle. If all we look at is fWAR (or RA9-WAR) and IP, Shields and Buehrle are almost twins. Buehlre got 4/$58 from Miami for has age 33-36 seasons.
4/$60 + some market inflation should be Shields' floor, assuming the market didn't overspend greatly on Buehrle (we've also seen lesser talents like Ervin Santana get something close to the Buehlre deal already this offseason). And as a "sexier" version of the same Buehrle type of talent, Shields can make a strong case for more money. A 5th year might be a major point of disagreement though between the Shields' camp and potential negotiating teams. But, if a team comes out and give's Shields 75M for four years of work, I'd say that that's basically his market value right now.