Ummmm...well, for starters, stick to land usage on this one because, that is what eminent domain is about: government taking of land for public use, not firearms.Your example leaves much to be desired for.
However, there is an actual answer to the question on value of land. First, understand this: the government can just take your land at any time. They can kick you off of it if they need to by passing an Act, which would bypass the courts. It is VERY rare that people win against the government in eminent domain cases in terms of keeping the government from taking your land. The Constitution is pretty specific on this one.
The REAL fight is over the value of the land. That is when the people have a far better chance of winning over the government. Normally, what happens is that the government sends out a real estate appraiser who will give a value to the land and any structures on it. The former owner will look at the value and decide to take or not. Usually not, because the government likes to under-value the property. So, the former owner has to hire a real estate appraiser to determine a value as well. That is normally higher than what the government will offer. Depending on dollar amount and need of the land by the government, the government sometimes will just cave if the dollar value is still low enough. Otherwise, it goes to court, which orders an arbiter to handle it and the lawyers haggle it...usually to the benefit of the former owner.
Courts usually rely on the proper value appraisal. Its on the government to prove why their number is lower than the actual value as dictated by the market. Which is possible, especially if the property in question is not well-kept (like the structures are condemned, the ground water is polluted, etc.). There is also the cost benefit to the land...for example, if the land is farmland...not only does the government have to pay you the value of the land, but the lawyers haggle out how many lost seasons to pay you out for. But again, that is reliant on the market....not what you or the government WANTS it to be.
Hope that answers your question. Just FYI, father is a retired real estate appraiser and a long time ago, I used to environmental appraisals (not a lot of money in that niche BTW).