Right now, the American public is looking at the cheapest RAM prices in history
Depending on your type of computer, you can get a 128 megabyte stick for $25 and 256 Megs for an amazing $45. Apple Macintosh prices are similar. New Macs (anything in color) usually can take standard RAM sizes and specs.
This does not include shipping and handling and you'll need to install the RAM yourself.
adapted from http://pcsupport.about.com/compute/pcsupport/library/weekly/aa080700a.htm
Primer: Random Access Memory (RAM)
Let's start with a description of what RAM is. Memory is just what
it sounds like storage space for your data. But keep in mind, in contrast
to your hard drive, RAM is short-term memory. When you turn the machine
off, your computer forgets everything in RAM.
What kind of RAM do I have?
BIOS - if you know how to boot into the window before Microsoft Windows, or quickly look at the screen before Windows starts, you might see the size and type of RAM in the computer. This way is not as reliable as opening the case (which you'll need to do).
Check with the manufacturer to see what type of RAM you need. The statistics are often listed in the bundle of papers and manuals you got with the computer. You did save them didn't you?
Crucial Memory offers a Memory Module Identification webpage - find it at http://www.crucial.com/library/memorymodid.asp
To indentify the memory, you'll remove the case cover and carefully note the memory slots available. You'll need to have one or two open slot(s). If need be - pop out a memory stick to see (or someone you know who is knowledgeable) what kind is it. You may not be able to tell If your memory stick had two slots in it, then you're in luck for the lowest prices.
I remember buying 2 sticks of 2-MB ram for $300, I was getting a good
deal.