I re-read the title of this thread and want to provide some food for thought....
'Least expensive' can be measured in many ways. High volume stores would probably argue that one of the large national companies are 'lowest priced' due to efficiencies in staff time, sending systems and added performance incentives like reduced member fees, rebates, credit card clearing discounts, website hosting and the like.
The answer is very reliant on volume and whether or not a business wants to fill incoming orders or be would be negatively impacted by the loss of the selection guides, holiday products, website hosting, credit card clearings, POS support or other services....
Some here will argue that none of those products and services matter. IMO the answer really depends on a shop's specific business model... the newer or smaller, the more likely that they can be easily changed.
I wonder if the response would be different if the question was posed as "Can someone recommend the most profitable wire service for membership?"
There's a wire service profit calculator
in this thread that may be of help in determining the answer.
BTW, David - thanks for catching my typo.