Depends on his health. Aint no one giving up as much value now as they will/would when word is he has good to go. Healthy? I'd give up 2 firsts and an additional pick. Maybe a conditional 3rd that could be a 2nd. Status up in the air? 1st and 3rd.
At this point unless you have a team built very well all around for the next few years I wouldn't give two 1st let's be real if you have no talent around and you want to get Peyton who is coming off an injury and is closing in on his last days I don't see the point in trading for him. Only reasonable scenario would be Peyton to Ravens as they seem to have it defensively along with running game.
Realistically at this point he's worth a 1st and some fillers.
Kurt Warner ages 36, 37, 38 tells me Manning could do the same. If docts after they fully checked Manning out and gave an honest assesment on his neck injury and the future? And it isnt as bad as many think or anticipate? I'd send 2 firsts for the guy. Carson Palmer brought back near that and Manning is way better. He hasnt lost anything as a QB, you have people bashing Palmer for his performance the last 2 seasons.