I looked it up and apparently the Dimension 8200 uses RDRAM, an early, long-discontinued competitor to DDR used in early Pentium 4s. Because it's no longer being made, or maybe it's being made but only in small quantities, it's expensive.
Like I said, not worth it. For $200 you could just about get a newer used desktop with a Pentium D (late dual-core Pentium 4), Athlon 64, AMD Sempron, or maybe even an early Core Duo or Athlon X2.
EDIT: In fact, Walmart sells a really cheap desktop for $130
. NewEgg sells the same for $140 + shipping so if you can pick it up instead of buying it online you'll save a bit.
It won't be a screamer... however, it takes DDR2, meaning you can upgrade it to 2-4 GB of RAM for really cheap. It ships with Linux (hence all the negative reviews on Walmart's site from idiots who bought it without looking at the box), but it's easy to install Windows.
If you want to move up a bit, more mainstream, powerful desktops should be available on Craigslist in the under-$200 range (as long as you're willing to negotiate), or you could probably build a really basic machine for about $300.