Almost universally, items under $25 that ship free with Prime are more expensive than on other websites and in local stores. Especially as you get into sub-$10 items I'd normally get at a grocery store or Wal-Mart/Target/etc. The shipping cost is being built into the price.
Stuff that's sub-$10 is either not offered with Prime shipping at all, or is only sold in non-discounted multi-packs. For example, say you need some deodorant. Amazon will gladly sell you some Old Spice Classic... but only in a pack of 6 for $18. I can buy just one at a store for $2.50 or so.
That's the value of Prime for us. When we need a pack of double-A batteries. Or Listerine. Or razor blades. Whatever it may be. It's a matter of supreme convenience and time-save for us to be able to just click, click, click, done. Especially so because our car sits in the garage most of the time. We can't just pop into target on our way home from work on the train. It would be a separate, special trip just for whatever little thing we need.