That's true, but without the 15% profit margin cap that wholesale clubs have. Lately I've found that certain Prime items (home decor and kitchen stuff) are 20-25% more expensive than what I can get them for locally.
At least on most of the stuff I've checked on CamelCamelCamel, Amazon has the lowest price on something maybe one day out of ten. Which is another way they push you towards Prime; if you have Prime, setting a price alert and buying it the day the price is the cheapest on Amazon is simple (assuming you can wait). If you're trying to combine items to get non-Prime free shipping, though, one item might be cheapest but the rest almost certainly won't.