Trade involving the top five picks often break away from the draft value chart by quite a bit (it's really the only section of the draft in which it does break down regularly by a significant margin). However, that's changed a bit over time.
Before the new CBA and rookie wage scale, teams drafting in the top five often didn't want to be there and have to deal with the rookie salary negotiations. So, they'd trade out at a sometimes substantial discount - 25%-30% in a couple of instances. Even so, there were plenty on instances in which a team holding a top-3 pick committed highway robbery and received up to a 30% premium.
Since the rookie wage scale has been in place, the "giveaways" have pretty much disappeared, especially when the circumstance of a particular draft have provided the high-pick holder with substantial leverage - the kind of leverage that the Giants have now with the #2. If they'll be happy with taking a QB later and feel they can acquire a high-value player at one position of need or another as long as they remain in the top-10 or even-top-15 after a trade, they'll be able to get every point that the chart shows for the #2 and possibly a bit more, because it won't matter to them if they end up "having to" pick at #2. It seems to me that about the only way they'd accept a "point discount" is if they're receiving five relatively early picks in exchange - or - as with any likely Buffalo offer - a pick in the top-15 plus another first (and more) that they can trade down for more picks in 2018 or even into the 2019 draft.
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As an example from this year's draft, the Jets acquired the #3 from IND (2200 points) in exchange for the #6 (1600) + both their 2nd-rounders, #37 (530) & #49 (410) ... a total of 2540 trade for a pick charted at 2200 ... they paid a 15.5% premium above chart values to move up 3 spots.