Back in the early days of personal computers, one of the most popular interfaces was MS-DOS, which stands for Microsoft Disk Operating System. Unlike competitors like Apple and Commodore, DOS could run on any PC no matter who built it, and users could even boot it up using floppy disks instead of a local hard drive. The size of the market also meant that a lot of developers made games for DOS. Not only could they count on a lot of potential customers, but computers could also handle more complex games than contemporary gaming consoles.

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It also helped that a few floppy disks were much cheaper to manufacture than proprietary cartridges. Of course, even if they were cheap at the time, some of those floppy disks have become collector's items. Like many retro titles, what were once regular games are now artifacts of history and nostalgia. The rarest and most expensive DOS games are far from cheap but are highly sought-after items for collectors nonetheless.

6 The Elder Scrolls: Arena ($710)

Back before The Elder Scrolls Online, back before Skyrim, and even before Morrowind and Daggerfall, there was The Elder Scrolls: Arena. A young company called Bethesda Softworks wanted to create a medieval team-battle game called Arena, but as development went on the game became more and more ambitious until it became an RPG about exploring an entire continent. The only remnant of the original idea to appear in the final product would end up being the game's name.

These days, Bethesda offers The Elder Scrolls: Arena for free as a digital download through Steam and their website, but physical copies are much harder to find. Between the game's groundbreaking scale and lasting legacy, it's no wonder that original copies have become valuable to collectors.

5 Neverwinter Nights ($787)

In 1988, the tabletop gaming company TSR cashed in on the rise of computer RPGs by licensing Dungeons and Dragons to the developer SSI. The games that resulted from this partnership are called the Gold Box series since most of the games came in gold-colored boxes. Neverwinter Nights was a special entry.

SSI partnered with the early internet company America Online (AOL) to create an online RPG that used the same engine as the rest of the Gold Box series, and anyone who used AOL's service could connect to other players across the internet. Neverwinter Nights wasn't the first MMORPG, but it was the first one to have graphics and not just a text interface. The Neverwinter Nights developed by BioWare in 2002 and the Neverwinter MMO developed by Cryptic Studios in 2013 are both continuations of this Gold Box game's legacy.

4 Commander Keen in Aliens Ate My Babysitter ($886)

Back before id Software changed everything by developing first-person shooters, the studio made a set of side-scrolling platformers starring Commander Keen, a child genius and space adventurer. The first set of Commander Keen games was popular enough for the developers to start their own studio, but the last few games in the series really brought the idea of the PC platformer to life.

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Aliens Ate My Babysitter is the final game in the Commander Keen series. It was originally going to be the last part of a trilogy, but the story and environments were unique enough for id Software to release it independently. Sadly for Keen fans, id would create Wolfenstein 3D in 1992 and the team never looked back to their platformer days.

3 Cosmology of Kyoto ($1,001)

Some collector's items weren't popular when they first came out, but they did have a cult following that kept their names alive long into the future. Cosmology of Kyoto is one such game, having gone from an obscure and somewhat bizarre title to one of the most expensive MS-DOS games in the years since its initial release.

Cosmology of Kyoto is a rare Japanese game that was developed for PC platforms, and its developer was called Softedge. The game has no central plot. Instead, players are dropped into Kyoto in the year 1000 AD and left to explore the city in a sandbox environment. The game didn't make much money in the United States, probably thanks to its unusual style and exclusively Japanese voice acting, but the film critic Roger Ebert gave it a glowing review in 1994.

2 Indiana Jones in Revenge of the Ancients ($1,280)

Indiana Jones in Revenge of the Ancients is a good example of a game that could only exist on a PC. The first adventure games ever made were text adventures, which meant that players had to type in their commands using keywords or letters and the game would describe what happens in text. Games like this are still made today, and the genre is called interactive fiction.

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Revenge of the Ancients is significant for a few reasons. While it wasn't the first Indiana Jones game ever made, it was the first one made for PC platforms. Next, it's the only interactive fiction game ever made using the Indiana Jones license. Finally, it's well-written fiction, which wasn't a guarantee even back when the genre was popular.

1 Commander Keen in Goodbye, Galaxy ($2,187)

Aliens Ate My Babysitter was a good entry in the Commander Keen series, but it's not the one most people remember. The entry in the series that broke the most ground and got the most attention was Commander Keen in Goodbye, Galaxy. The game featured relatively refined and colorful graphics that made the cartoony nature of the story come alive, not to mention the excellent sound design and the creative set of enemy types and level designs.

The team over at id Software would put this creativity to good use again when they developed Doom in 1993, and there's a reason more people remember Doom than Commander Keen. Still, Goodbye, Galaxy is a fun game, it's a historically significant game, and it's a rare game to find in-box thanks to id's old shareware marketing model. Put together, it's really no wonder that it ended up being one of the rarest DOS games.

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