Go to a sports memorabilia shop and ask. In all honesty, it sounds like you might have a fake. Peyton Manning knew from day 1 what number he'd be wearing and that number was #18. Don't believe me. Google for pictures of him on draft day. He held up a #18 at the draft. Ryan Leaf was the one that wore #16 and he went to the Chargers. And a Ryan Leaf signature would be worthless.
Also, you might want to check who the certificate of authenticity is by. Just because it has a "certificate of authenticity" doesn't mean it is real. Because sometimes the certificate of authenticity can be fake too.
Sometimes a person advertising something as a rare misnumbered item is their way of covering up a botched fake. Think about it. The item might have been faked around the time that Peyton was drafted. And Peyton and Leaf were both in the news. Both numbers were on the tv at that time and the faker might have easily forgotten which number went to which. Because most thought the Colts would pick Leaf.
I'd have someone look at it to see if it is a fake and if you got scammed. Compare your Peyton Manning signature with actual Peyton Manning signatures. See if it looks the same. And see if it looks like the person wrote slowly or fast. If they wrote slowly, it is likely someone tried to copy and it is a fake.
Signed footballs tend to go for less than signed helmets. Also where it was signed makes a difference and how readable the signature is. Sometimes signatures fade too. Even if it is authentic it is likely not worth over $400.
What really goes for the big cash is if you own something that has been worn in a game by the player and it is certified by the NFL itself. Something like that would go well over $10,000 for say a jersey of Peyton Manning's that has been worn by him.