Unfortunately, there is no standard percentage. Each case is decided on its own facts. It depends upon the extent to which the land owner can still make use of the land that is subject to the easement.
Expropriation is a complex process and few land owners have the necessary knowledge and experience to deal on a level playing field with an expropriating authority. Owners usually need independent advice from expropriation professionals to get the best results.
In many cases the cost of professional advice will be more than offset by the additional compensation that can be recovered with the professional's assistance. In addition, owners may be entitled to reimbursement of part or all of their professional costs. A lawyer experienced in representing property owners in expropriations should be able to answer these questions for you.