Part of the price is because it is slot loading. Most laptops use tray loading drives thus not as many slot loading ones are built. Especially the really slim drives needed by Macbook Pros.
Powerbook medic does carry one for the low price of $700
http://www.powerbookmedic.com/xcart1/product.php?productid=17116
WOW! That's still too much!
Mine's the older MBP 'classic' 17". There's space to use standard 12mm height HD's and optical drives.
Slimmer 9mm drives are only required for the MacBook, MacBookPro15 and possibly the new Unibody MBP17.