Why do batteries cost so much?

Couldn't find anywhere else to post it, and I suppose it can go technical so yeah. To start, I have a Dell Inspiron 1501. My battery recently started popping up messages saying it's nearing the end of it's usable life(with 2 hours average time, 50% wear, 9cell 85Wh), so I thought I'd take a look at how much they cost, it wouldn't hurt having a spare brand spanking new battery in case of an emergency or something like that. The page loads, WHAT! £84.60 For a new , *6* cell, 53Wh battery? Roughly $172.710 USD, for you Americans. But, that's not all, I literally, shat bricks when I saw the 9 Cell, 80Wh(notice 5 less than what I got now?) for a third of the price I paid for my laptop?! £113.98, or $232.665 For a battery that will last no longer than 4 months using a laptop the way it's suppose to? So, Dell, why do batteries cost so much? Another forum I use, don't like Dell for one reason, being overpriced, I've never had a problem, until now. I understand that making safe batteries is expensive, but the one I've had has just worn out so fast, it would be cheaper and more efficient, possibly even more mobile to buy a desktop pc. I've also seen some on popular online auction sites claiming to be brand new, and genuine offering serial numbers for over £80 less than what I can buy direct from Dell. I wasn't kidding when I said a replacement battery to meet the specs I got now costing 1/3 of the price for my laptop, I bought my laptop for £350(friend, brand new), and can buy it for £400 from dell.com directly.