Original Xbox FAQ
Why are original Xbox disc drives so expensive?
There are multiple reasons
There are actually several reasons why the original Xbox disc drives are so expensive. First of all you have to understand the console generation of the time. We had the original Xbox, Gamecube, and Playstation 2. The PS2 DOMINATED that generation. In fact, this generation has never had a gap as huge as the one Sony made in the console wars. The Gamecube was a great console but never sold well, and the Xbox was Microsoft's first attempt at the game console market. It was expensive, it had fewer games, and therefore was a very low selling console. So this is our first issue. There isn't an abundance of them in the first place.
Replacement drives were never available to the public, though some options did exist. This means the drives you can find now are only drives that currently still work or can be refurbished. This also means every year that goes by, more and more of these disc drives break down. However, This is still the same case for many other gaming consoles over the years, so this is only part of the issue. If you take this knowledge and combine it with the modding community on the console, you can start to see the whole picture.
The modding community
The modding community is a HUGE aspect of why original Xbox disc drives are so expensive. The original Xbox spawned an entirely new modding scene that had never been seen before. The design of the console made the capabilities endless. To this day the original Xbox has one of the largest modding communities of ANY console from ANY generation. Now, what does that have to do with the disc drives? These modded consoles can cost hundreds and thousands of dollars. Mod chips and custom mod kits with special features can run hundreds of dollars alone. Then imagine having huge libraries of games on upgraded 2TB hard drives with tons of game saves from emulation and homebrew. Then consider all the time it takes to build one of the consoles. When you put that much time and effort into a build, and have thousands of hours of gameplay and game saves invested on the console... It doesn't make since to buy a whole new consoles and start over just because the disc drive goes bad. This makes those disc drives worth a lot more to people because they are keeping their time intensive and costly build in working order.
So when you combine the low availability mixed with the modding community you have the perfect storm for a rare part that a lot of people want and that a lot of people are willing to pay a lot of money for.