At the per-pill level, opiate painkillers can sometimes be even cheaper. According to StreetRX.com, a site run by epidemiological data firm Epidemico that crowdsources street price data on a variety of pharmaceuticals, individual pills of hydrocodone or oxycodone can be had for as little as $1 depending on which city you're in. That's roughly the price you'd expect to pay for a Snickers at your local convenience store.
On the topic of cheap drugs, let's not forget that alcohol is the drug of choice for many Americans and it's extremely cheap. One-dollar drink specials at your local restaurant or liquor store aren't that hard to come by.
The other implication of Trump's statement is that drugs are “becoming” cheaper. From a long-term perspective, this is also true. Federal data show that the price-per-gram of heroin, cocaine and meth have been dropping precipitously since the 1980s. The typical purity of those drugs, meanwhile, has increased over the same time period.
The notion that we're a “drug-infested” nation, meanwhile, is a bit of Trumpian hyperbole that's of a piece with Trump's harsh rhetoric on all things drug and crime. There is a kernel of truth here: Opioid overdose deaths have been on a relentless uphill climb since the late 1990s. In 2015, for instance, heroin overdose deaths surpassed gun homicides.