Without throwing in
more crazy go nuts 8s and 9s type things ...
you reach a point where the exp burn has you down a level for a little while.
So the real cost is 2 8th level spells, and 1 level behind for some part of a level.
Some of the full casters are getting 2 9th level spells at leveling up.
This means the
trade is 2 8th level spells and 2+ 9th levels spells, for a few feats.
When you catch up the level the trade is 2 8th level spells, for a few feats.
Of course CO-ing can mitigate such small problems ...