Final answer:
The least costly type of prescription drug for the member using the drug benefit is the Generic RX.
Step-by-step explanation:
The LEAST costly type of prescription drug for the member using the drug benefit is the Generic RX. According to the information provided, the cost of Generic RX is $15. This is the lowest cost compared to the Name brand RX options, which are $50 for Preferred Name brand RX and $70 for Nonpreferred Name brand RX.