Yes, by the month is more expensive. It's $99 for the year. If you do a lot of on-line shopping you'll find shipping rates on other sites to be anywhere from $6 up! I shop a lot at Amazon, and would otherwise spend the $99 on shipping (from other sites) in the first month!
I use Prime for movies and TV (free...not current programs, but programs that I otherwise wouldn't search out, or don't have that particular cable channel). And movies -- I haven't been to an actual theater for years, but watch movies on my Kindle all the time. And free books -- you get one free book a month, from a choice of 4-5. There also are occasional book sales for prime members. And then free fast shipping with purchases. Yes, there are a lot of food items available through Amazon, and the Amazon grocery store. You do have to fill a box at the grocery store to get free shipping, but I always fill it with stuff like toilet paper, or paper towels that are bulky to carry around a regular store, anyway. A prime membership can be extended to include anyone in your household that uses the same Amazon account, so in my case Tom Cat can order and get free shipping, too. Even Consumers Reports, which is pretty picky, says the Amazon Prime membership is a good deal.