One of my favorite new additions to the PFF Draft Guide this year was the draft capital ranks and five-year allocation charts. This was based on a study our analytics team did to properly evaluate how much each pick is worth historically in terms of expected wins above replacement (WAR).
The value of a pick decreases rapidly at the top of the draft but then flattens out over time. The first overall pick is worth 1.13 WAR on average over the first four seasons, while the 15th overall pick is worth 0.67. You can see the full curve below.