You're not comparing like for like. Of course you could go even grander than that, spend £20 on a bottle of red to put in it, use only the most expensive free range steak from the butchers and mince it yourself, add a load of gold flakes to the top, and spend £60! or even £100! But when people say it's cheaper to make stuff from scratch they mean if you make an effort to use cheaper ingredients 😂If you can afford to spend £18 on a spag bol then the advice isn't really aimed at you. It's aimed at people who could really do with saving that extra 50p.
My spag bol (serves 6):
500g of minced beef: £5.50
a pack of dried pasta: £1
grated cheese £1
Those are all things you'd buy whether you used a jar from Aldi or make your own sauce. Same with garlic bread, so don't include that in the price.
For the sauce:
2 tins of the cheapest tinned tomatoes: 66p
dash of Worcester sauce: 10p
squirt of tomato puree: 30p
dash of red wine vinegar (we only crack the red wine out for special occasions): 20p
1 carrot: 7p
3 pinches of various herbs: 15p
onion: 12p
garlic: 15p
comes to £1.75.
For 6 people you'd need 2 jars of Aldi sauce, so £1.50.
OK so it's 25p dearer for me to make it from scratch. But it is nicer and healthier. If I was really broke I could ditch the red wine vinegar and two of the herbs, and just use oregano, and save myself 30p. Mine also has fresh veg in it.
Also Spag bol is just one meal, and probably the very cheapest that you can find in the supermarket. I don't think you can get jars of curry sauce that cheap, or chilli con carne sauce, or casserole sauce etc. So perhaps in that one, specific instance it's almost as cheap to buy ready made, but for the rest of your meals I don't think it is.