How to 'stretch' tomato ketchup?
I think your answer is found by responding to the question: "Why do I make my own ketchup?"  Heinz ketchup is delicious.  It is also produced in mass quantities.  You'll never compete on cost because of economies of scale. If you are purchasing ingredients, homemade ketchup is just going to cost more.  This brings me to the question I posed.  If your answers are "it's fun"...."I like the flavor"...."I prefer my ketchup over Heinz"...anything along those lines, you may just have to accept that yours will cost more to produce. Maybe find a way to buy tomatoes in bulk? You might get even closer if you grow your own tomatoes. Even then, the cost would likely be more when you take all of the production variables into account.  Any "stretching" will change your recipe, probably at more cost, possibly at risk to the flavor you have developed.  If you are trying to make ketchup to save money, there are probably more impactful home financial strategies you could employ.