wow its looking to be like 60 bucks or so for 4 512 sticks. pretty ridiculous. The computer is junk so it would not be worth spending $60 on. I would spend $30 but there is nothing for $30 besides 2 512 sticks that i can see
DDR3 memory is the only affordable memory right now. As older memory tech is phased out of new machines manufactures stop making the older memory in large volumes so it's prices skyrocket after the next memory technology overcomes the previous generation.
Your best bet would be looking through Ebay but even then it is unlikely you will find a good price on old computer memory.