When thinking about the cost of Prime, you need to think also of the cost of defaulting to Amazon for many purchases and not doing any price comparison. I have found that now it's fairly common to find the same item significantly cheaper elsewhere, but I just go with Amazon because of the convenience. But this convenience has a higher cost than just Prime, imho
I have noticed this with our purchases as well. Because of Prime, most of our online purchases are Amazon-first. Only when they don't carry it do we check elsewhere.
We recently cancelled because of customer service issues with Amazon, but we are close to signing up again. I'm on the fence, but my wife just assumes we'll be back on the wagon, that it's our 'default state'. It makes me wonder whether that by itself is good or bad.
This is my observation as well. Often you can find the total cost including shipping is at or under the Amazon Prime cost with free shipping. However, most of the time this means you will have to take "standard" instead of two-day shipping. Is it worth $10/mo?