When you buy tomato sauce you are getting a pre-made tomato sauce rather than tomatoes. It's something you can heat and add directly to pasta, it's not an ingredient in a sauce, so it's going to have garlic, herbs, salt, etc. If you want 'unadulterated' tomatoes to make your own sauce you want to get canned whole, crushed, chopped or puree.
Making a basic tomato sauce yourself takes minutes, I personally never buy a pre-made sauce as there's no point, you're spending extra money for something you can do yourself, and better. A bit of olive oil in a pan, fry chopped garlic for about thirty seconds, then put in a can of tomato puree (or passata in the UK), a pinch of whatever herbs are on my mind that day (usually oregano and/or basil) and a few twists of pepper. Bring it up to a bubble for 2 minutes, and it's done. It takes less time than to boil the pasta.
With tomatoes you get what you pay for. Cheap tomatoes have added water (usually in the form of tomato juice, which is red water with a hint of tomato flavor) to reduce their cost, and often have additives to make them seem more tomatoey. Most grocery store chains will have decent canned tomatoes, provided you can shell out for them. Many tomato products have acidity regulators, every whole canned tomato I've seen has added acid, I expect this is because there's more water in the end product so it needs some extra.
For recipes that matter, in the UK I wait for Mutti tomatoes to come on sale and then buy loads, but I haven't seen that brand in the US. In the USA I buy Cento if I can, with Tuttorosso a close second. If you look at their cans of chopped or pureed tomatoes you'll see one ingredient: tomatoes. However, if you can't get or afford those a cheaper can with a dollop of paste will give you a decent result.
I would not recommend basing a sauce on reconstituted tomato paste, as paste has a lot of tomato flavor cooked out. But, if that's all you have you can make it work.
