Because it aligns incentives with total honesty: you pay for intelligence at cost plus a visible 5 percent, so Falkster profits only when Heidi does real work, and has every reason to make her efficient rather than chatty.
Flat AI pricing hides a quiet conflict: the vendor wins when you use less compute than you paid for. Cost-plus inverts that, waste hurts both sides, efficiency helps both sides, and the meter is public.
It also future-proofs the deal: as model prices fall, and they keep falling, your bill falls with them, automatically, instead of the savings disappearing into a vendor's margin.