Who's selling it and more. For instance Dell doesn't get their RAM off the spot market but may buy it when the machines were made, pay the price then and not adjust prices as time passes. Crucial.com has a higher inventory turns so it's price could be lower. Then we move to companies that only stock the 20% of what models comprise 80% of the market. These 20/80 vendors will be names such as Newegg or other and the price is lower due to volume but you may not be able to find RAM for your machine.
Hope this explains it for you.
I'm looking to upgrade the memory on my computer,Iknow which I need but am wondering why there is such a difference in cost between makes?
The specs are the same so what would the difference be?