What does 'buying a planet' get you?
For nontrivial possessions, a significant factor in market price is the set of rights (both de iure and de facto) that you get by 'buying' it.
For example, real estate grants you (mostly) exclusive rights to use and develop it - but the price is significantly affected by the extent of those rights. For example, in our current world:
* Are mineral rights included with the sale? Will the government in that place impose a 'resource tax' taking half of the value of the oil/diamonds/whatever produced there?
* Will someone enforce your rights? In particular, if others start exploiting the land that you bought, will the government or seller enforce your rights, or have you simply bought a paper that says that they won't interfere with you, but it's your own problem to actually claim, enforce and protect that land?
* Are there any limits to the exploitation? Suppose the land contains an important and valuable transit route; are you allowed to take tolls/taxes on that traffic for crossing your land or is that traffic free to cross? Even more, maybe there is a duty for the landowner to maintain that route at their own expense while letting the traffic pass freely (e.g. medieval bridge maintenance in some parts of Europe).
* What about the current inhabitants? Are you allowed to evict them or charge arbitrary rent?
* What about limits to allowed activities - owning a nice plot of fertile land is rather useless if it has been designated a nature reserve where farming is not allowed.
This goes double for buying planets.
* Do you get unrestricted rights to all natural resources - can you strip mine the whole planet if you want it to?
* Do you get autonomy/independence - i.e., can you make your own social laws without interference "from above", or do the people on that planet have the rights granted by some other government?
* How about current inhabitants - are there any, and what rights do they have after you've bought their planet?
* If the planet exploitation gets de facto contested, will you have to (re)conquer it yourself or would buying it also buy the enforcement of your rights there?
All these factors will change the value of 'buying' planets significantly, simply even if the planet is the same, you're buying very different things depending on answers to those questions. 87