I'm not an on-site kind of visitor, but I think you can plug in your dates of travel and whether you want only hotel or hotel and hoppers on disneyland.com and it will give you rates. I'm sure others know better than me though.
I didn't have any luck finding the room rates for the on-site hotels either. I ended up doing what the previous poster suggested, which was plugging my dates in for all three resorts and finding out the prices.
FWIW, Paradise Pier was the least expensive, Disneyland Hotel about $400 more, and Grand Californian $400 more than that.
We booked Disneyland Hotel, but I recently switched it to HoJO...for right around $104/night, I think. Whatever it is, it's saving me $630.00.