I think it's a fair deal for him. The backup position is really important in football. The best-case scenario is that he doesn't have to play at all. However, it almost never happens that way. Only a really small percentage of teams have the luxury to play their starting QB in 100% of the snap during a season.
Siemian is 6"3 220 pounds. He signed with the New Orleans Saints last season. He played in 6 games and started 4 of them. Although he didn't win a game, he played actually great. In those 4 starts, he has 1154 passing yards, 11 touchdowns and 3 interceptions.
He showed great upside and also a good ability to adapt on the fly. This is crucial for a backup QB because you are one snap away from playing live football on every play.
Check out his arm strength on that throw. Serious zip on that ball