I'm not sure if this is the right sub to ask this in, but I was just pondering earlier today and figured that there are currently a lot of long-standing border disputes that could be solved pretty quickly if someone just flashed some cash...
Related question: when was the last time a significant chunk of land changed hands between two nations? Maybe Alaska?
The type of transfer you are looking for is called a "cession", and specifically a cession for payment.
Although money was not exchanged in the transaction, the most recent negotiated cession of territory between two sovereign nations that I'm aware of would be the cession of Macau back to China from Portugal in 1999. Although the Chinese did not pay cash money to the Portuguese, they did give several concessions regarding the governance of the territory. Given that some of these were economic in nature ("Due regard will be given to the economic interests of Portugal and other countries in Macao."), it could be viewed as a cession for payment.
The value of land and people is so how now that this would not likely be able to solve anything recent
For instance considering russias current financial straights its extremely unlikely they could afford to purchase crimea and/or eastern ukraine because the price would be 100+ billion dollars considering the long term value of land and production
The main issue though is most country(in the western world atleast) have become quite homogenous and taking on more people for a huge amount of money is not politically tenable considering what else could be bought with it and the cost of integrating these people into a new country especially when in many circumstances they dont want to leave their old one
Pretty much all the examples of buying and selling territory are of colonies, not of what might be termed 'the motherland'. The territorial colonial period has come to an end, and so has the purchasing of territory largely.