Generally speaking, top picks that haven't signed – especially this late in the process – are haggling over contract language, not money.
In fact, the value of every rookie contract for an NFL draft pick is more or less predetermined based on where the player is selected.
Instead, players and their agents are often negotiating over offset language. Essentially, that means how much the team will have to pay if they cut the player before the end of his deal and he's picked up by another team.