When talking about cap space at this point in the offseason, it's important to keep in mind that teams have to set aside room for their rookie pool. Each year, teams bring in several rookies via the draft, and you have to factor in the cost of adding them when making plans related to cap space.
For the Detroit Lions, for example, Over the Cap projects that they will have a rookie pool of around $5.1 million for 2014. This may make it seem like most of their current cap space is going to be set aside for rookies, but that's not actually the case.
In terms of the salary cap, Over the Cap projects that the Lions will need to set aside only $2.6 million or so for their rookie signings. That number is much lower than the actual cost because only the top 51 contracts count against the salary cap during the offseason. By adding draft picks, players with cap hits of $420,000 will be bumped outside of the top 51. This means that if your seventh-round pick has a cap hit of $435,000, for example, adding him only costs you $15,000 in cap space (during the offseason, anyway).
Of course, the Lions can't just think about their cap situation in the short term. They will have to be under the salary cap with all of their players factored in when next season starts. Also, the Lions have to account for their restricted and exclusive-rights free agents. Even so, with around $5.1 million in cap space right now, according to Over the Cap, the Lions have approximately $2.5 million to work with after accounting for the rookie pool. That's not much, but at least they're actually under the salary cap right now.
(Note: The current rookie pool projection doesn't include compensatory picks, but adding a fourth-rounder for the Lions should take away only $50,000-60,000 in cap space.)