The problem is that the only people who can say how much money is necessary is the landowners themselves. However, they have an incentive to lie if doing so would allow them to get a higher price. Additionally, different landowners would value their own land at a significantly different rate relative to the market. For example, if you live in a custom built house, with a treehouse that your kids built themselves, that is within walking distance to the school and your place of work, you would need significantly more compensation them most people to move out of your house. I don't see any solution other than letting cases where an agreement cannot be reached go to court.
There are three people are well-placed to say how much money is 'market value':
1) The owners themselves, by self-declaring asset values for taxation purposes (on pain of having them vulnerable to eminent domain seizure if they under-declare)
2) Independent assessors who do value assessment for property tax purposes already
3) The sellers that granted the owners possession for a certain value
