Copper bullets penetrate MUCH better than comparable standard lead bullets. If someone is using a caliber normally considered borderline for the game hunted they can make a huge difference. For someone who normally only deer hunts with a 243 and gets an opportunity to hunt elk or bear it is cheaper to upgrade the bullets rather than buy a bigger gun. And there is a good argument to just using the same bullet and load for everything even if it isn't needed all the time.
Cost is a relative thing. I hand load. The cost savings between 200 of the cheapest bullets I can buy, and 200 premium bullets won't pay for a tank of gas in my truck. 200 bullets will last me several years. The cost of 1 tank of gas a year is nothing in relation to the rest of the money I spend on hunting.
