The 5 pack saves $1 a season, which saves $5 for 5 seasons, which is what you are buying. And either way, it's less per season. If you don't think you will receive any rewards credits, then it makes sense to get the slight discount, otherwise it's better to use credits when you can.
In terms of $8 per month, the math works out. A 1 game a day world lasts about a month and a half. So 5 seasons * 1.5 equals 7 months. $59.95/7.5=$7.993 per month.
I feel like when they wrote how much it was a month, 2 a day worlds may not have even existed yet.