Checking Alibaba superficially, one finds one available with the following characteristics: Motor: electric; Payload: 5kg; Range: (60 sec @ 18m/s = 1080m/min = 0.67mi/min) * 90 min = 60.4 miles, for about $5,000 to $7,000 depending on extras. Such minis can fly at under 100 feet over all terrain. Its silence, tiny cross section and low mass would make it invisible to most forms of detection, again especially at night.
The 1,000 miles of the US-Mexico border that is not regularly patrolled, especially at night, provides a landing zone of 60,000 square miles. Discounting for really remote places, that leaves smugglers with millions of random landing sites where recipients need be on site for only a few seconds to receive a delivery, and whose locations and arrival times are coordinated in real time on their GPS-enabled smart phones with accuracy to a few feet.
The wholesale value of 5 kg of heroin at $28/g (ten typical addict doses; rehacenter.net) means that the shipment is worth about $140,000. Subtract the cost of the heroin in Mexico ($25/gram) for a total of $25,000, and we have a gross profit of $115,000. The delivery cost amounts to a couple of "burner" cell phones @ $100 and the helicopter, which is abandoned after its one-time trip, with all components having been handled with rubber gloves all along the way; good luck NCIC!
After distribution and cost of sale, the street value rises to about $82/g (other estimates are up to $280) for a total of at least $400,000. (Heroin values from CBS News, 24 Jan 2019.) Assuming the lowest estimated number of addicts in a brief search (400,000 @ 50mg/day = ), that shipment represents about 1/20 of US demand. So about 20 guys with little helicopters and a huge incentive can overwhelm thousands of CBP (border patrol) officers.
Note the various sources use different rates of consumption per addict, making precise calculations difficult. What is not difficult is reaching the conclusion that anyone who thinks a wall will make any difference to smugglers es loco.