There really isn't an answer for this because I have seen some quote on quote "gaming pc's" go as cheap as $5 using old and free parts. I am sure you could build a system for free with old and free parts. However, where do you draw the line of what is a gaming pc and what is not? On the contrary, as far as expensive, you can go with the best parts and more than what you need, plus custom water cooling, and reach $50000+. However, at some point your going to start using parts that aren't really gaming parts as their for other tasks like 3D rendering and professional workspace environments. You can go to pc part picker and pick the most expensive stuff there is, but are they really gaming parts? For example, cpus that have oem on their name, isn't a gaming pc and is for other tasks. We can prove this by comparing an i7-7700k that is $350 and a oem that lets say is $1500 and see the i7-7700k win every time in gaming but when it comes to 3D rendering, lose every time. So as you can see, this really isn't a clear thing to ask.