Wow, camping on Cape Cod has gotten expensive!
I've stayed at this place and it's fairly nice.
I've been told that the MassAudubon society (massaudubon.com) has (or at least used to have) a really nice campground on the cape. You might contact them and ask them about nearby good campgrounds.
291 State Highway, Route 6
P.O. Box 236
South Wellfleet, MA 02663