If you make the reservation for three nights at once, I think it's $50 per night and you get charged the $6 fee just once. Yeah $156 is a lot for three nights on the river, unless you have a big group to split the cost with. If you aren't going over the weekend you can make the reservation for all three nights at once. The weekend spots are taken quickly and probably need to be booked separately as they become available.
This will describe the campsites rather than having to do research for each site. Hope that helps!