Do we know the pricing difference (or even the ability to add) for the new bundles if added after you book the flight? (i.e. book the basic fare, but add the next level sometime after booking)

Not seeing bundles available on booked trip, just the ability to upgrade seat/bag individually.
https://cimg1.ibsrv.net/gimg/www.fly...fde0589476.png

In other words, you can't avoid the Carrier Interface Charge AND save money by getting the bundle for the lowest price. You are going to pay extra on something one way or another...

By the way, I was able to get the 2500 mile one way awards for a CLE-LAS roundtrip in early June. So they're not necessarily being restricted to the absolute pits of the travel calendar. But I agree with Rusty that these ultra low awards will not be plentiful during anytime with even moderate demand.

You can, you just need to do it at the original time of booking. Can't add it retroactively.

My one way flight was $39. Economy bundle is $69, Premium bundle is $79, Business bundle is $114. I'm seeing $49 flights have $10 increases to each bundle, and $59 flights having $20 increases to each bundle, so it seems like the bundle price is standardized to whatever the base fare is.

Just got an email about there being a bunch 2500 mile awards for a "limited time." Also included in the email was 2500 mile bonus for those who purchase a flight between 5/30 and 6/30. These are travel miles only, and interestingly enough you can book each leg of a trip separately and earn 5k miles, as this promo bonus is limited to two awards per member.

https://www.flyfrontier.com/2500-travel-miles-june