The difference in money between a 3rd round pick and a first round pick is huge. That's why many players who could enter the draft stay. To put things in perspective, Sam Bradford was paid a six year 78 million dollar deal. The Rams 3rd round draft pick, Jerome Murphy, signed a 4 year 2.7 million dollar contract. BIG difference! Even an early second round pick like Jimmy Clausen (the 2nd QB off the board) was paid a 4 year, 6.3 million dollar deal. Money from the NFL draft is exponential, so moving up improves your payday a ton.
This year's draft class is pretty deep at quarterback, with a 3-5 legitimate first round options. Entering the draft would be a pretty bad financial decision for Pryor right now. And if he sits out those 5 games, those are 5 games where he can't get hurt. All he has to do after that is put together 5-6 good games, and his draft stock will go up.