The bus company doing the Madrid - Valencia route is not
Alsa/Enatcar but
Auto-Res . Their buses don't depart from any bus station but from the
Auto-Res station at Fernández Shaw, 1, besides the N-III (Madrid - Valencia road).
The fares quoted by Javier are promotional fares and you have to meet certains conditions to get it. The standard fare is more expensive.
Besides, think that the plane takes 50 minutes whereas the bus takes 4 hours. However, the airport is located further from the city centre than the bus station (that means you'll spend more time on the transfer) and you have to be at the airport at least 1 hour in advance.
Auto-Res Express buses have seats that are more comfortable than Airbus-320 tourist class ones. You have more leg room, toilet, free newspaper, courtesy earphones for the music channels / video. The price is 39 euros round trip. Normal buses are 33 € (but don't include the facilities above).
A more expensive alternative to the bus is the
Altaria train . Altaria is a fast train and one of the best we have in Spain (with facilities similar to the ones at the AVE). Prices are nearly twice the bus ones (around 60€ / round trip in tourist class) but it's faster than the bus (3:30 hours).
My advice is to take the plane for longer distances (eg. Madrid - Barcelona, Barcelona - Seville) where the time you save flying is worth the money.