Where can I see the prices for entering a particular rally?

The most reliable spot to find the prices is in a document call the Supplemental Regulations. Every rally in the country publishes this document, and all of them include the prices to enter that particular rally. You'll find them in the "Competitors" section of the web site for that particular event.

Additionally, you'll want to consider that you need a NASA membership and a NASA Rally Sport rally license, these two together total up in the neighborhood of one hundred dollars.