You can probably get 256Mb of memory for less than �20.
But note it is VERY important to get the correct memory for your motherboard.
Memory is changing all the time, with different number of pins, different speeds and so on. If you buy the wrong memory it will stop your PC working.
This web site has a program (see system scanner) which will tell you what type of memory you need.
Note that sometimes memory from 2 different companies may not work together. It MAY be better for you to buy a 512Mb (or even 1Gb) and replace the memory you have.
Once the new memory is working you could put your "old" memory back in to see if it works. If it does it is a bonus, if not you can just take it out and you will still have 512Mb.
Check the documentation that came with your computer to see how much memory your computer can take (some have a maximum)