You're right, if you're looking to spend less than $500 overall on a low-end desktop, you'll probably never be able to build something that isn't unreasonably outdated compared to the retail options at a similar price point. Grabbing the latest off-the-shelf model from Walmart is likely the easiest thing.

Building reasonably performing computers is another thing. My worry is the following: there is almost no substitute for live testing when it comes to physical hardware. This is because unless you replicate make, model, OS patch-level, and driver versions exactly there is very little guarantee that the performance or reliability of a given configuration will be what it you read it should be online.

There are sites like these guys (www.hardware-revolution.com), who do a great job offering mostly-tested full builds, with mostly-tested alternatives depending on your end goals (power consumption, noise level, etc.). However, the difference between component A and the slightly more expensive component B might be 5% improvement, or 20%, or one might be completely at odds with the sound card and corrupt the hard drive on a weekly basis (true story).

Components themselves might not range into the industrial laser-cutting price range, but if you make fairly simple mistakes, you can easily misspend $2000 on something that either doesn't work, or that you aren't happy with for the price you paid.