One thing that should be mentioned here is that a more premium diet doesn't just equate to a potential longer lifespan, it typically also equates to a much healthier, stronger, and more disease resistant fish.
Some of the bioactive compounds found in the more "premium" fish foods have been shown to have biological effects in fish such as growth promotion, immunostimulation, anti-stress, anti-bacterial, anti-fungal, and anti-virals. These substances will work in a positive manner no matter if the fish is 10 weeks old, or 10 yrs old.
It's not just about longevity .....
When I choose a feed for my dogs, I don't just consider if they will live to 13, or 18 yrs of age, I also factor in their quality of life, while they are alive.
As far as cost, one can't just compare sticker prices either, the true value of any food is dependant on that foods nutrient value, and it's overall digestibility. In commercial aquaculture the difference between a low feed conversion ratio, and a high FCR, can equate to hundreds of thousands of $$$$ in feed in a single year. Cheap food does not always equate to savings.
There's a reason why generic farm feeds cost a significant amount less than a more premium fish food, the same as why a low cost supermarket brand of dog food costs a significant amount of $$$ less then a more premium brand.
Like most things in life, you get what you pay for.