"Admittedly, the battery in the MacBook Pro is more powerful than the Air's: Apple rated the former at 95 watt-hours (Whr), meaning that it can produce one watt of power for 95 hours, or, say, 5 watts of power for 19 hours. The 13-in. MacBook Air's battery, on the other hand, is rated at 50 Whr."
Not that a 90% increase in battery capacity should have any impact on the replacement cost or anything...