I guess comments are discouraged as answers, so I've moved this to an answer.
You could stretch it with Heinz (or other brand, French's maybe) ketchup. If its a taste thing, it would lower the quality but still be better than store bought. Since you're looking to lower sugar/corn syrup content, try reducing the sugar in your recipe to compensate for the extra sugar in the commercial products. You could even remove it entirely and adjust upward from there.
Alternatively you could try beef or chicken stock, heavily reduced, but this would cause significant changes to the taste and you may still need to adjust the water content and add a thickening agent. If you are making your own stock, pork bones can often be found for an exceptionally low price.
You could also try similar vegetables that are cheaper than tomatoes. Throwing in a few onions could stretch it a bit farther. Hot peppers could add some spice and make it go a bit further as well.
That also sounds very expensive for canned tomatoes. I've gotten them on sale for 28 oz for $1-$2 CAD in the past (this depends on your location, of course). Consider shopping around if you haven't, or switch to a cheaper brand (you can doba taste comparison to see if the quality is similar). Amazon currently has canned tomatoes for $0.13 USD/oz, so consider an online source as well. Otherwise you can probably can your own tomatoes when they are in season for cheaper.
Not directly part of the answer, but I also found this article from the Washington Post that might provide some insight:
Americans didn't always have a singular idea of what ketchup should taste like. Indeed, the first ketchups weren't made from tomatoes at all. According to "Pure Ketchup," Andrew F. Smith's definitive history of the condiment, the first known English-language recipe was published in the 1727 edition of E. Smith's "Compleat Housewife" and called for anchovies, shallots, white wine vinegar and spices such as mace, ginger, cloves and lemon peel, making it closer to an Asian fish sauce. By the early 19th century, cookbooks offered a range of recipes for ketchup made with walnuts, mushrooms, lemons, cucumbers, oysters and, of course, tomatoes.
Heinz's recipe is a trade secret. But some of the brand's earliest recipes are not. Its first blend included cloves, cayenne pepper, mace, cinnamon and allspice. The second had black and white pepper, ginger, mustard seed, horseradish, celery seed and brown sugar. Vinegar and salt were added to taste.
You could consider adding more ingredients to boost the flavor, and then water and a some amount thickener to stretch the amount of final product.