Around here prices for 87 are roughly 10 cents less, maybe a little more at times. The big savings is premium 93. its 40-50 cents less. That coupled with being top tier makes it the best choice for anyone who uses 93. For me that's roughly 3,000 miles of using their fuel would pay for the membership.
Zero actual evidence of this, but my old jeep 4.0 seems to run better using costco gas vs wawa.