A recent article in the Washington Post cited StreetRx, stating crowdsourced data that indicates that

individual pills of hydrocodone or oxycodone can be had for as little as $1 depending on which city you’re in.

Similarly, a recent article in the St. Louis Post-Dispatch, stating:

according to crowdsourced street drug pricing sites like streetRx, it is possible — albeit rare — to find a single pill of some prescription drugs for [one dollar]