does the math really work out?
You will also have to spend quite some money on fuel and should factor in the depreciation and maitenance costs of your car to make it comparable. And flights on popular routes in Europe are pretty cheap nowadays.
Estimated cost both ways is about 600EUR, that includes fuel, tolls, a cheap hotel half way. Brussels - Bilbao is always an expensive flight for some reason. There's Ryanair airports nearby but it's a huge hassle and we'd still have to rent a car upon arrival.