I'm no fan of the patent system, but "patent system promises spoils for coming up with great new drug, companies comes up with great new drug, companies gets spoils" is exactly how it's supposed to work. Yes, it's sucks that you have to pay, but that's how you incentives getting the wonder drugs invented in the first place. (I have my own take on this, but if anything this is a 'textbook case' in favour of the patent system.)
Eminent domain would still require fair compensation to the company, so you'd have to pay them more or less what they'd lose from not having the patent anymore.
(Though I think the term you might be looking for is 'compulsory licensing' or so? Not sure.)