Refer to Thomas Davis reply, but I think tent sites are around $12/ nt.
Go to their site and click on reservations. Look at what is available that has what you want. You can do the whole reservation and get the price including the fee for reservation and leave before checkout without paying, and will know if site is available when and as you want. Prices are very fair and include the day fee for park entry.
Depends on the site you choose. I believe there is 3 different prices for different types of sites.
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