Sometimes, the greater good trumps individual's rights. Case in point, when in my house, two kids want to see the movie downtown and one doesn't, the protesting party is drug along to the theater with promises of popcorn to smooth over hard feelings for having to participate in an activity they don't want to, because the rest of the clan does in fact want to see whatever is on the big screen. Eminent domain is kind of the same thing, if the government wants to use your land for the public good (i.e go to the movies) and you can't agree to let them take it or agree on how much the land is worth (i.e not go to the movies), then a compensation system in set up to determine what size of popcorn you get.. er I mean...how much money you will receive for your property.
First, if you voluntary agree to let some government authority take control of your property or some private person do the same, you are avoiding eminent domain, not participating in it. In this piece, read government to include whatever private company (like a utility) is trying to use the eminent domain power.
Interestingly enough, if you voluntarily give up the easement, you should ensure that in the event of non use or abandonment, you get the property back. That is the way it works if the government takes it from you, but if you voluntarily give it up, you may not have this protection unless you work it into your agreement. If the agency is offering you 130% of whatever value is derived from the appraisal they order in addition to paying a portion of your fees, it is doing so to meet a "good faith" requirement that is required to be met before it can forcibly take your land.
Iowa Code Chapter 478 outlines how we play and share well with society in eminent domain situations. First, you need to remember that private property can be taken for the public use if it has a demonstrated public purpose and you are paid "just compensation".
The rub is usually on just compensation. First, however, a demonstrated public use has to be found. In the case of power lines, which is a hot button topic lately, the Iowa utilities board (which you didn't elect) decides if the power company has demonstrated a public need. For public purposes, the government has to establish that a need exists and that the easement, or taking, will not unreasonable interfere with the use of your property. You have a right to object to this finding.
Once the public use is established, another proceeding covers how much will you be paid. This procedure is covered in Iowa Code Chapter 6B. The chief judge of your judicial district (which may not be in your county or somebody you can vote out of office) appoints a commission of six individuals. These six are picked from a list of 28 people, selected by the Board of County Supervisors to determine what to pay you. The commission gets paid $200 per day plus actual expenses, on the dime of the government agency, for doing the condemning.
Then, the sheriff arranges for the six to meet, at a time decided by the government, they view the land and determine damages. The important part is the hearing that follows the onsite inspection (which could be skipped with consent conceptually). That's where each side trots out an appraiser to give testimony on the value. As an aside, if the commission awards more than 10% over the last offer by the government, the government has to pay the land owners attorney, appraiser and expenses. The commission votes at an open session public meeting. If you don't like the result, you can appeal to the district court