Only counting the PC itself -- not the output (can be expensive, or can just use an existing TV) or input (negligible cost) devices -- probably around 500 USD.
That's if you build your own, prebuilt is more expensive.
I'd argue that the cost of entry is higher but there are other factors which can compensate for it such as no yearly online subscriptions to play online, average cost of buying games on PC is cheaper (Steam, GMG, etc.), etc.
I stated this in another topic but I'd like to see someone create a thread on GAF on the advantages of PC gaming and going into detail on each point. With the turn of a console generation a lot of "platform rivalries" tend to start, and you see more misinformation or simply misunderstanding of PC gaming.
Take fps (or more accurately frame times), which is one of my major advantages of PCs, the difference between 30 and 60 is immense, if we look at 120 even more so. It isn't just the perceived smoothness of the image in motion, it's the controls as well. At 30 fps there is a noticeable delay in input, which causes the whole experience to suffer.
This is an interesting app that lets you compare, obviously you're limited to comparing up to your monitor's refresh rate.
http://frames-per-second.appspot.com/
The fact that it is an open-platform you have the option to choose/change/customize everything at your disposition. If you want to disable Bloom in a game because you find it unbearable? You can. The option of being able to go into the .ini file and change anything from disabling mouse smoothing, enabling graphic settings not present in the in-game options, removing v-sync from the engine, etc. If a company decides to shutdown the game servers? Often (depending on the game obviously) the fans will put one back up. Put it simply, you're in control of your experience, it isn't decided for you.
For me personally, and arguably, the majority of game innovation, whether in the form of game play, concepts, etc. has come from PC. The Indie scene, mod communities, or even something like Kickstarter has brought about many new ideas, real gameplay changes, or improvements on existing genres. For many genres the PC is the only option, or others exist in a very simplified "dumbed down" state on other platforms.
I feel a lot has already been said in this thread, but a more in-depth detailed discussion on each point could help attract or simply inform players of the platform.
I'm open to hear any other low-spec, quality games that my laptop can run (Battlefield 2??)