Sure, APF 2K8 would be worth $79.99 to me, and most 2K fans. The question is: At what price do you get the more casual gamer to buy a generic/customizable game?
I expect no more than a $49.99 price at release, at the very most. A $39.99 price would be even better, as a $20 price difference with Madden will make many think twice. A $10 difference isn't much of a buy/not buy factor for most people, but $20 is.
Now, if 2K would like to recreate the aggressive strategy of 2K5? To sell so low that most football gamers are tempted, even AFTER getting the new Madden game? Then $29.99 would do it.
Even the "Platinum Hits" are selling at $29.99 now, instead of $19.99. I'd love to see them run a deal with Gamestop and other places to let people trade in Madden 08 for a copy of APF 2K8!
The biggest obstacle in most mind will be the lack of a NFL license, but at the right price, and with created rosters here and there on the internet? It will be up to us hardcore 2K fans to show APF 2K8 to everyone we can, and what can be done with custom options. A job made MUCH easier when the game can be had for $30.