It's expensive because almost no one makes it anymore as it's been out of use for years. The process is also more expensive, and it's an extremely low volume part.
You already know what to look for on Ebay " 184-Pin DDR SDRAM DDR 400"
Honestly it's time to replace it and memory isn't going to help your performance. Pentium 4s were bad when they came out, let alone now. I don't even think most of the prescotts can handle a 64bit OS so you might not even be able to use 4GB. No sense throwing money art it when you could replace it for 2 or 3 hundred with something much better