Just researched a bunch of Copper Age keys comparing the supply of Newsstands to Direct on Ebay listings and also looking at the CGC census.
The higher end was Amazing Spider-Man 361 first Carnage with 35% Newsstands listings (35 out of 100 listings checked) and a census of 15,000.
On the lower end was Batman 404 (2,299 census) first Modern Catwoman and Wolverine (1987) 1 (15,436) with 6% to 10% Newsstand listings.
The average was around 20% Newsstand listings.
Definitely seems like a noticeable supply difference.