Lurker > Waddlez

Grocery sushi for me is usually MORE expensive than restaurants. I've seen prices of 8-12 dollars for a normally about 6-8 piece sushi. When I can get like 6-8 pieces for 4-7 dollars for plain rice seaweed fish.

ANOTHER thing, usually the option for grocery sushi is more shelf stable sushi so stuff like california rolls made with imitation crab which doesn't "spoil" like fish does. Or if there is you might get lucky with shrimp sushi and if you're REALLY lucky MAYBE salmon or tuna. Regardless odds are you're not getting the best cuts of salmon or tuna.

If you go to a restaurant that actually has pride in its sushi you're likely to have better cuts of salmon with more or less fat depending on preference and if it's a sushi restaurant worth its salt in ANY manner, you'll be fortunate enough to see that they have several tuna varieties. Here's a picture so you understand.
https://gamefaqs.gamespot.com/a/user_image/8/1/6/AABn_6AAEcIo.jpg

Most sushi in america will be sold to you as Akami tuna which is the LOWEST quality part of tuna. It's usually super purple and has not too much fat in it. In fact you won't even know this, they'll just call it tuna sushi and move on with their life.
If you're fortunate to have a sushi restaurant that has some pride in its work, you will actually see more variety to their tuna sushi. You may see options like toro tuna, chuutoro tuna and ootoro tuna. These "toro" types of tuna basically mean fat. The closest comparison is quality fat kind of similar to how kobe beef has tons of fat in it compared to other beef types. It's not often you see JUST toro and if you do, it's KIND OF a warning sign that they don't distinguish from chuutoro and ootoro. But it does mean that they acknowledge that it is different from just standard akami tuna, basically odds are that tuna cut will have SOME fat somewhere and will be of better quality than akami tuna but you won't know how much.

https://gamefaqs.gamespot.com/a/user_image/8/1/7/AABn_6AAEcIp.jpg

Chuutoro tuna means middle/medium fat, so basically moderate amounts of fat. Chuu in japanese kanji means middle. So think of middle tier.
Ootoro means literally big fat or in this case high [quality implied] fat. It has a lot of fat. The oo in japanese kanji means big/high. So think of it as the highest tier of tuna you can get.

In my opinion the cost of ootoro to flavor isn't spectacular, you're better off going for chuutoro. There IS absolutely a difference in flavor between all three.

I used to be like you, I only ever had sushi in most of my local places which I thought were great. Then I went to Japan and realized what I was missing out on. There's a significant difference in quality of fish and I never even knew there were even types of cuts to a tuna. I just assumed tuna sushi is tuna sushi no matter what and some/most restaurants that offer sushi in America are banking on the fact that you're ignorant to this knowledge. Most of the sushi places around me (I don't live close to a coast) don't even offer chuutoro or ootoro, which just goes to show are they REALLY a sushi restaurant if they don't offer such a basic option? Do they even KNOW about chuutoro or ootoro? Who knows. The assumption is when you go to a sushi restaurant they're the ones carving up the actual fish to serve to you, but instead odds are they just get the cuts sent to them and that's that, no quality tuna sent in or served to you.