I voted for the $80 buy-in option. You will see profit if you finish top3 or higher, and having payouts for 4th, 5th and 6th place is nice too (since FF has a large luck factor, I think it is good to reward people who do well in the regular season but get unlucky in the playoffs). Also, the payout to the winner is still pretty good considering how many places are getting paid.
I almost voted for the $100 buy-in as well because the top prize will give you the highest profit percentage, but it still pays out 4 places.
Personally, I prefer leagues that offer a payout to the team that finishes the best in the regular season as well. There were a lot of teams last year that were stacked with LT and some other good players, but didn't win their leagues because they ran into SJax in the playoffs. Also, I don't particularly like leagues that payout weekly high-scoring prizes. It rewards teams that have a high degree of luck and decreases the prize pool at the end.