Say someone paid the MacBook Air 11" for RM2600 brand new.
The battery is one of the 3 most important component that keep the Mac running, besides the logicboard and display. If we split the unit up part-by-part, the battery replacement cost RM600-700 is merely 20~25% of what the whole Mac cost. Expensive? Maybe a matter of perspective.

If you are not paying for quality parts when you purchase the Mac. What else you paying for? The logo?