Sadly, like the cost of all health care, the cost of prescription lenses has skyrocketed. The closest you get to prescription cheap reading glasses is comparison shopping optometrists. Costco and Sam's Club, however, offer prescription discount reading glasses for members.
In general, however, you get what you pay for when it comes to prescriptions. Buying prescription reading glasses wholesale would be like buying surgery wholesale. Costco doesn't offer prescription bulk or wholesale reading glasses. Save your bargain hunting for sunglass sales and not for a prescription reading glass sale.