In fact it costs less for you to buy a single in a shop than it cost the shop to buy it. At least for the first week. The first week is the big sales week, and that means record companies want to push as hard as they can all week. This started in the mid-90s, right about when the chart went haywire, not as a way to make the first week the most important but ironically to try to give singles a lift in the second week. The reasoning was that a cheaper single meant more sales, so for the first week there would be bigger sales.
More sales equals higher chart position.
Which theoretically meant more airplay the next week, and so big singles could be inflated by this trick. Gradually we've reached the point where this doesn't work anymore. Instead all singles are heavily discounted for the first week, and so that's when everyone buys them. The small bands on small labels who can't compete. I don't know.