We have attempted to engineer it such that archiving one web page costs about 1 ARC, but this rate will ultimately be determined by the market of miners, so we will have to see.
With trillions of web pages now existing, that seems like quite a high fee, IMO. When first reading about this project and the projected supply of ARC, I imagined it would cost more like 0.001 ARC per page. As you indicate though, the market will eventually dictate the price.
Good luck to all with this exciting new project!