Yes and no. The cost of moving the plane through the air is the same. There are some minor on-bard differences in costs in terms of food an beverage and other amenities, but those are relatively low.

The main reason that the airlines dn't just give away the seats up front is that if they always did the chances of selling those seats - either for cash, miles or some combination thereof - diminishes. So the airlines don't like giving them away for free unless they need to.

The airlines will oversell the coach cabin if they know there are unsold Biz/First seats, as they know that the OpUp is a tool they can use, but they won't just fill the front cabin every time for the sake of giving out free upgrades.

Looking at it another way, it doesn't generate them any extra revenue so why should they OpUp you? Long-term goodwill is something, but it is very hard to quantify.