Techniques and ingredients to make marinara sauce from canned spagetti sauce
If for some reason, you only have canned goods available in your area, I would start with canned tomatoes -- either crushed, diced or a mix, depending on how chunky you like your sauce.  (some people would argue that whole peeled have better flavor, but you end up with a can with a lot of juice in it so requiring more cooking (or reserving the juice for minestrone or some other purpose)

If canned spaghetti sauce is all that you have, or you're trying to use up a case, you don't necessarily need to cook it down for a long time.  You can instead pull the pasta early, and finish cooking it in the sauce; the pasta will then soak up some of the moisture thickening the sauce.  You may want to reserve some of the pasta water in case it ends up a little too thick by the time the pasta is cooked through.

As for spicing, yes you can modify things if it's bland, but if it's an off flavor ('tinny') or something too strong, it's more difficult.  Sometimes a little bit of sugar can help if it's a touch sour, but you can also cook down some onions or finely diced carrots and mix that in to get it some  extra sweetness (and texture, and other flavors)

And if the issue is price differences -- If you have a Wegman's near you, try their store brand sauces.  It's like $1/jar (about the same as what a can of sauce costs), and it's quite good.