Ah, I see your point, but it doesn't make much sense as a comparison, since you wouldn't be listing 40 items
for a month, you'll be listing up to 40 items
over the space of a month.
To display one item for a month on auction costs 4 x 0.50 = 2 PED (plus starting markup %).
To display one item for a month in your shop costs 1 PED. So it's half the cost, assuming a minimal starting markup. You can save more
if and only if you sell and replace items within that time, or have an item with a large mark-up.
I guess it could get quite complicated after that
PS Come on England!