Collectible football jerseys are valued based on several factors: whether they were worn and by which player, whether they were autographed, and what era they're from. A jersey autographed by a famous player and a worn jersey autographed by that player are two completely different things to collectors. Many players autographed limited numbers of promotional jerseys, which were often sold with a certificate of authenticity.
The other big factor is era, either for the team, or within the individual player's career. Numbers no longer in use by a football team are more valuable, especially those from the team's infancy or a big winning season. For individual players, jerseys from college years are often sought-after.