I thought about Britney's touring the other day and realized that she made $200M with 176 dates since 2009. It's actually really good and an average of $1,136,363 per show. Not bad.
I personally believe that album 8 will fair better in sales than FF. I came to this conclusion because i'm pretty positive that the promotion for Vegas and Album 8 will go hand in hand. Rumors are saying that the official vegas announcement may be made at the end of this month, I wouldn't be surprised if a single release date was also announced or more info on album 8 is revealed. All the press from Vegas will hopefully work wonders for Album 8 AKA Dancing on grapes with Jesus.
Well, considering the fact that Las Vegas shows are generally seen by literally millions of people over their lifetime, it could significantly help sales in the US if the show is really good.