Not sure the $$ numbers would expand all that much with an 8 team playoff. This is because that could really push the 'traditional' bowls off of Jan 1 and even the big ones would not be considered remotely premier anymore. While the CFB playoff brought in an additional $200 million, the non playoff bowls brought in $300 million to bring the total to $500 million. That's in a non Rose Bowl/Sugar Bowl year. Those are going to pay ~$35 million (per conference) next year, and that will escalate as the TV contract continues. So P12 will get ~ $50 million from the CFB playoff, and $35 million from the Rose Bowl (but only in 2 of 3 years due to the rotation). Next year the BT, SEC, P12 and B12 all gain a contract bowl. The BT/SEC and ACC however lose a contract bowl with the Orange going to the semis. That still means an extra ~$70 million due to non playoff bowls, so the payouts would nominally be $570 million total, $200 million for the CFB playoff and $370 for the sum of the non playoff bowls. In 2016-17, all 3 contract bowls would be played, so that pushes the total to $640 million, with only ~$200m coming from the playoff. Of course in 2017-18, that resets back to the low figure. Probably the best way to look at it is an average of the three years, which would be about the $370/$200m split between non playoff bowls and playoff payments. Both are likely escaating at the same rate, by the way, as is the P12 regular season TV contract.
So if the format moves to 8 teams, and the additional Rose Bowl payout goes away, it's not going to be all that much $$ extra for an 8 team playoff - and there might have to be a lot more sharing with the smaller conferences while the Rose Bowl (or Sugar Bowl or Orange Bowl) goes directly into a P5 conference without anyone else watering it down. If the system makes the round of 8 permanently located at the traditional bowls, there will not be a double payment, just a larger one for the additional round.
Now when the current contract is up, maybe the $$$ will make more sense. There very well could be quite a lot of erosion of the Rose/Sugar Bowl brands by then due to skipping years and having multiple non-champion matchups. If people stop watching the New Year's Day Bowls, I'd think ESPN would be eager just to throw money and push tradition side. ESPN did screw up by paying for the semis on Dec 31st in 1 of 3 years, but why should the P5 change to suit ESPN (without $$)? There are still other networks to bid though, so if the Rose Bowl and Sugar Bowl are well attended and well viewed, that just might be more profitable to continue next round of TV contracts (at least for the P5).