Alexei Miller, the head of Gazprom, has explained that the reason for raising gas prices for Ukraine is in part that Ukraine no longer is entitled to discounts that it received in return for leasing the Black Sea fleet base. According to Bloomberg:

In Kharkiv in 2010, Ukraine agreed to extend Russia’s lease to the Black Sea Fleet base in Crimea from 2017 to 2042 in exchange for cheaper gas. Russia has no need for the accords after the peninsula’s accession, Prime Minister Dmitry Medvedev said last month, calling for Ukraine to pay about $11 billion lost to Russia’s budget.

Of course, the lease is void because Russia now owns Crimea! The original purpose of the contract has been frustrated, as a contract lawyer would say. You can’t lease property from yourself. But shouldn’t Russia compensate Ukraine for the loss of a chunk of its territory, if we’re going to be legalistic about this? (Miller’s better argument is that Ukraine forfeited its discount by failing to pay prior debts.)

Joseph Blocher and Mitu Gulati argue that Putin should have bought Crimea rather than taken it:

Perhaps if Mr Putin had negotiated to buy Crimea instead of taking it over, Ukraine could have negotiated for both debt relief and multiple years of cheap gas in exchange. Russia might even have helped the current Ukrainian government track down some of the funds that the members of the prior government supposedly absconded with. On the flip side, there would not have been any need for all the chest beating, troop movements, and so on. And the international community surely would’ve been more likely to bless the result—a result for which Russia might be willing to pay some premium.

But why pay for something that you can take for free? Anyway, the chest beating seems to have been the major benefit for Putin. And if a sale was really in everyone’s interest, there is nothing in international law that would have blocked it.