Free agent contracts are generally poor investments for baseball teams. Long-term free agent deals usually do not turn out well. Players with long-term contracts signed close to free agency generally work out, like the current contracts of Adam Wainwright and Yadier Molina. Players in their arbitration years, like Trevor Rosenthal and Brandon Moss, are generally solid values. Long-term contracts signed before arbitration, like Matt Carpenter, and even Jaime Garcia, are generally solid investments (Allen Craig excepted). And of course, players making the major league minimum, like Carlos Martinez, Michael Wacha, and Kolten Wong are downright bargains.
Free agency is for filling holes a team cannot fill internally, like Jhonny Peralta, or signing an impact player like Matt Holliday. Out of all the ways to acquire players, free agency provides the least relative to cost. There are exceptions. Matt Holliday has been a great value for the Cardinals as has Jhonny Peralta, but those are exceptions. The Cardinals front office felt David Price and Jason Heyward were excpetions as well, but just because the club missed out on those two, that does not mean the team needs to set their sights lower and still spend a ton of money without making significant improvements.
There are good players out there in free agency. Alex Gordon, Justin Upton, Yoenis Cespedes, and Chris Davis are all good players, they are all going to be expensive, and all of them have likely already played their best baseball. Upton, the youngest, is just 28 years old, and he might be worth the $125-150 million contract that he is likely to receive. He might provide solid production for a few years, but there is considerable risk that as he enters his 30s, he will be a below average player. Matt Holliday is projected to be an average to slightly above average player at Age-36 this season in the final year of his contract, and that borders on miraculous.
Over the last five seasons, 472 position players have put up 2-WAR seasons or greater. Of those, roughly a quarter came from players over the age of 30. Raise the age to above 32, and the number drops in half. Move up another two years and the number drops in half again. Chris Davis and Yoenis Cespedes will be 30 at the start of their contracts while Alex Gordon will be 32. All of those contracts will be paying for the downside of their production.
Repeating an exercise done in the comments recently (by tehzachatak), let's take a closer look at the impact any one free agent hitter will make on the team next year. Let's start out with what the Cardinals have now. The team has virtually every spot on the roster filled with a single starter except for right field and first base. At those two positions, we can estimate around 700 plate appearances and let's take another 100 plate appearances in left field when Matt Holliday has the day off. Those three splitting 1500 plate appearances, and according to the FanGraphs Depth Chart projections, they should total around four and a half wins above replacement. It might be a little more or less depending on your level of optimism or pessimism, but that seems like a pretty good bet.
Continuing with the free agents, the four players above all have projections ranging from a bit under three wins above replacement (Davis) to 3.5 WAR (Gordon). For the sake of argument, let's say those players are worth 3.5 WAR next season and take up 600 plate appearances. Assuming the other players are now worth 2.7 WAR, the Cardinals just went from 4.5 WAR to 6.2 WAR, an increase of 1.7 WAR. The next season, with Moss gone and Piscotty still around, the new expensive free agent is not blocking anyone even if Matt Holliday's option is picked up, and then when Holliday is gone, that free agent pickup in his third year still has a role and still is not blocking anyone even if another player steps in. Looking at it this way, we see an increase in production to the team and a role in future years. I can almost talk myself into it. But there's a catch.
That 3.5 WAR the team got in the first year of the deal is not constant. It is three wins above replacement that next year, then 2.5 WAR, then two, that marginal value you were getting is gone, and the contract is tough to trade because the money left now outstrips the future value. All of this assumes that the player is worth 3.5 WAR in the first year and declines normally. That big power of Davis and Upton do not age well, Cespedes can't take a walk, and for Gordon the aging gets worse the further a player gets into the 30s.
The Cardinals have money. We know all about payroll muscle, 3.5 million fans, the new $1.5 billion cable deal beginning in a few years as well as five straight postseason runs. The Cardinals have a ton of money, but that does not mean they should waste it, or spend top dollar on marginal increases. There is a dollar value where I would take Upton, Davis, Cespedes (who has expressed a willingness to play right field in the past), or Gordon (maybe $80 M for Gordon, $100 M for Davis or Cespedes, and $120 for Upton), but unless the market drops, those players are unlikely to be decent values. If the Cardinals can add a low-end free agent pitcher at some point for reasonable price, they should do it, but any long-term contract is not just an investment for next season, it is an investment for the long haul. If they are doing that sort of investment, it is probably better spent retaining flexibility for a trade or locking up players like Carlos Martinez, Kolten Wong and Michael Wacha past their arbitration years.