Difference between Lithium Polymer batteries of varying capacity and cell amount
What you are missing is the voltage. You're paying for energy storage, not for mAh. If you put n cells in series, the mAh does not change, but the energy stored is n times as much, because that's how much more voltage you have. Since they're in series the current (mA) through each is the same.

Some batteries are rated in W-h (watt hours) which is directly proportional to the energy stored (1 W-h = 3,600J),