When you have different assumptions about the future, it's hard to agree on valuations.
If a football team disagrees with a player about how much he's worth as a quarterback, one solution is to use a contingency contract. In other words, if the player produces the stats laid out in the contract, he'll earn his higher perceived valuation, while if he doesn't, the team only pays their lower counter-proposal. If the player is confident in his abilities, he should be willing to take the contract because his payout will be the same if he can deliver, which is his rationale for the higher valuation in the first place.
So, if you're having trouble finding agreement due to differences of opinion, try using contingency contracts to allow everyone to "win" when their assumptions about the future turn out to be true.