If you're coming from Spokane, is it fair to assume you're traveling via hwy2 and taking 174 north at Wilbur? This puts you off route for most of Banks Lake, where there is lots of free drycamping at WDFW-managed public access areas...
I've camped at Buffalo Lake on the Colville Indian Reservation- about fifteen miles northeast of the dam. Fifteen bucks a night, no amenities...they do have hookup sites but I don't know the rates.
Here's an article about Buffalo Lake.
ON EDITFound the current rates- twenty bucks now.
Here's a link to the "resort" that I camped at- you'll note that it's cheaper to rent a CABIN than it is to rent a full-hookup Rv spot! I think the cabins have no inside plumbing... The whole place is a hoot- real throwback to the camps of my childhood.