Gee, you get to just hand-wave an entire manufacturing process and supply chain into insignificance, but I need a model, huh?
Fine. Let's use a generic drug (ie, not paying patent/development costs) as a proxy for how much the entire supply side costs. How about loratidine. You can get that at Walmart (stocked on the shelf, since that's mostly what the article was discussing) for about 10 cents/pill.[1] Of course that's completely ignoring the disposal costs. I strongly doubt that disposing of hospital/pharmacy quantities of drugs is as easy as chucking them in the municipal landfill. Let's say it costs 10% of the purchase price. So 11 cents/pill just to run a pill through the supply chain and then dispose of it.
Intensifying the napkin math... CDC says there were ~3.4 billion visits that resulted in drugs "provided or ordered". [2] Let's say that those were an average of 7 days worth of drugs ordered, so ~30 billion doses. Let's say that 10% of that much is either wasted by the consumer, or wasted while sitting on a shelf before anybody bought it - 3 billion pills.
3 billion pills/year * 0.11 cents/pill = $330 million/year. I dunno, seems potentially significant, at least to an order of magnitude level. Especially since the costs of the FDA program to extend the shelf life was around 1% of that much.
Ref:
[1] - https://www.walmart.com/ip/Equate-Loratadine-10-mg-300-ct/20...
[2] - https://www.cdc.gov/nchs/fastats/drug-use-therapeutic.htm