Super cheap? Goff earned $28m dollars on his rookie deal with another $18m in signing bonus on top of that.
Not to mention not every rookie quarterback will have a Carson Wentz type of debut.
Wentz numbers so far - 1,324 yards passing 8 TD pass 3 INTs QB rating of 92.7
Goff numbers so far -
goose egg.
With Garappolo, he's already broken into the league and learned to be at the starter level. Depending on who you take, it might take a couple of years before they are ready to start for a team and actually be good enough to win football games for you instead of losing them.
So there's value both ways depending on how you look at it. A team that trades for Jimmy Garappolo is getting a guy ready to step in and start from Day 1. He's already had the training and breaking in period and he's still really young. If you're looking to turn around a franchise faster, JG is a better bet than an unknown rookie. Especially if you're not picking #1 overall.