A patent is property manufactured by the government that ceases to exist if the government ceases to exist. It's not clear why a government would buy a patent, they may as well just pay a corporation for nothing. Better off funding research, which until now the government does, then NOT granting such patents (or granting patents but making research funding contingent on pricing control). Buying a patent by the government is merely invalidating that patent, something a government can do for free, theoretically.
But in the case of our government and our health care system, reducing problems in the population is arguably not the goal. Besides, a more universally effective solution would be single payer, it would give purchasing power to that payer to negotiate prices, regardless of the patent. This drug is expensive in the US because of our corrupt system, it is much cheaper elsewhere WITHOUT buying patents.