This is a question homesteaders often ask themselves, and how hard core a homesteader you are, how much money you have to spend on groceries, also how much free time you have will often determine the answer.
This morning, for instance, I processed about 20 pounds of tomatoes for tomato paste. I've never made my own tomato paste before, and since a) my tomatoes are still producing like gangbusters and b) I've already canned the amount, plus a little extra, of regular tomatoes I will need for the coming food year, I decided to take a stab at making my own paste.
Out of 20 pounds of tomatoes, blanched, skinned, de-seeded, stemmed, and strained well, this is what I ended up with. By the way, I left the Mason jar with kitchen compost in the frame so you can get the general size of the bowl.
It was about 3 hours of hard work, for one half bowl of paste. And this isn't even the end of the processing. Tomorrow, I will need to cook this down, reducing it even more, before it can really be called paste. And then it will need to be canned in a water-bath canner.
So regarding whether it's easier and/or more financially worthwhile to make it vs. buy it, I am guessing it's probably easier and more cost-effective to buy small cans of organic tomato paste at the supermarket, and use all your tomatoes for canning. After all, regular, old home-preserved tomatoes are great for making spaghetti sauce, in stews and chili, and you could even make tomato paste out of your own preserved tomatoes, real-time, if you wished.
Along a similar line, there is a fantastic book out there I just finished called, "Make the Bread, Buy the Butter," by Jennifer Reese, which talks about this exact subject. Part cookbook, part autobiography, part short story collection, it's a wonderful look at what things are better made at home, and what things may just not be worth it. Tomato paste comes to mind.