There is a excellent – albeit in French – Youtube video about it: L'HISTOIRE DE FORT BOYARD P1 - Les années Laffont | Le Télévisator. Yes, that's almost 2 hours long, quality Youtube content right there.

The story is quite a bit of fun. The fort was purchased by a dentist at an auction during the 60's. For the price of a very nice car: about 50,000 euros (adjusted for inflation).

When it got the interest of a very succesful TV producer, they had quite a genius move. They purchased it back from the dentist for 500,000 euros (adjusted for inflation) – nice markup mister dentist. It was then sold to the region Charente for 1 euro on the condition they finance the restauration (quite expensive, the fort was in poor shape) and agree to rent it exclusively to the production company.

Today, the fort is a symbol of one of the most succesful TV game ever created and is quite a staple of french television. This is also a good move on Charente part, even though this property is very expensive to maintain, it is a huge tourist attraction. Which is fun because before the 80's the existence of the fort was mostly unknown apart maybe from fishermen.