There’s a campsite on the lagoon of Obidos North of Lisbon. It’s on search for sites and they rate it one of the best. 4€ per night, or 6€ with leccy. It’s at Foz do Aurelio.
We stayed there in September and loved it. Probably the best site for the money we have ever found. Don’t tell anybody!
Obidos is a lovely walled town with a free aire too.