pc computer used under $500.00 for music only

CAPS V3 Topanga. Check out computeraudiophile.com, look into the C.A.P.S. Tab/menu on top, and scroll down to the Topanga.
This is if you want to buy new.

You can probably get a used CAPS V3 of the better ones for about that price too (this is just a guess, though).

Lenovo Q190. I'd recommend replacing built-in HDD with SSD for extra $75.

BTW, you haven't told us anything about your system, or your level of comfort with computers.

Obviously the easiest route is a laptop. But the best results come from a headless computer, along the lines of the CAPS, which also allows you to upgrade some key parts of the computer that in turn will deliver better sound (ie, the two CAPS v3 models that are based on the same mobo: Lagoon and Carbon).

The absolute best results come from the same concept but with better mobo (motherboards and hence other parts too). My server is similar to the CAPS v3 Zuma, but with better parts, fanless, headless, very optimized for audio. It's a huge improvement compared to a laptop I used to use running the same software. I'm no computer wiz, but I wouldn't recommend this route for someone not at all comfortable with computers, though.

So depending on your comfort level with computers, you can spend 500 on a laptop for ease of use, or on a CAPS type for better sound. Do you have an iPad/iPhone, android phone, or laptop that you could use as control device for the CAPS? The control device doesn't need to be dedicated to audio, but it is certainly advisable to run your laptop/CAPS server as dedicated for audio, running no antivirus, or other processes that are not essential for music reproduction.

Sorry I got long-winded. But not knowing where you are coming from is difficult to know if my suggestions are applicable or way off base for your case.

And also keep in mind that it is quite possible to get a very capable brand new laptop for that kind of money. Check out BestBuy, NewEgg, TigerDirect for great deals on new equipment...

-RW-

Depending on the horsepower you need you can get a nice Atom based PC for less than $500. Zotac makes nice ones. Atom processors with hardware acceleration can easily handle 1080p movies let alone high res FLAC files. I personally use two of them for my audio though I will be making a CAPS in the near future for my main listening room where I want the best sound quality.