Of course they cost something, but, depending on how much you use the service, the majority of expenses is shipping.
But Prime saves me money, no more running around to different stores to buy certain things, since I work from home , great time saver.
One item is a certain Dog Supplement for my yellow lab because of her gastric issues, the only store that has it is about 40 min away, also a lot more expensive, so I buy it from Amazon, save the $20 more and 1.5 hours of my day.
By the way, it only comes as a month’s supply, so $20 saved times 12 is $240, a hundred dollars more then the price of Prime.