Except the medicines themselves aren't expensive. The prices are that high because the pharma companies demand it, because of all the tests, which are their main source of income. That is why prices in the US are so much higher than elsewhere (because of the political power of lobbying and cultural acceptance of price screwing the public on medical things).

Given that it's the very same party that would be doing the testing for expiration dates, I kind of doubt they would research this out of the goodness of their corporate hearts. Effectively the same outcome could be realised without the effort of extra testing anything: by just lowering the price of medicine a bit; big pharma would make less money, the public would have more medicine for cheaper.

There may be other ways to approach this from a moral point of view. High prices for medicines are defended on the basis of high research costs, not high manufacture costs.

A case could be made for legislation that obliges free replacement of out of date medication for the original purchaser, perhaps with a small surcharge.

A case could be made for smaller packages so people are less likely to take out of date medications as well.