I searched my usual sources for memory and that was what I found. Either those high prices or No longer available.

Maybe you can find someone who pulls old memory and re-sells it.

Considering the system you have, and the best used price on Amazon was one set for $90 the next for $114.

You could go up to 1GB total for much less, maybe $90. As others have said, always in pairs. Add two 512 chips and you have 1.2GB of memory up from your 512 total. Double plus a bit.

Go on Craigslist and search for Desktop Computer. I think you'll be surprised that you can find a Dell GX series, Pentium 4 processor, with memory, LCD, Windows XP pro and who knows what else, for $200 or under.