It's very difficult to get grip on the estimates price of a "simple" LAMP server. Here are some numbers to help you get an accurate estimate of a LAMP server.
So as a conclusion: One average webserver costs around $ 120,- (with tax) per month to run on the EC2. There are cheaper solutions to host your website(s), but Amazon EC2 provides the option to add a machine in a couple of minutes or even automated. This flexibility is not found in many other products.