Taxi occupation detection

Hello,
Maybe someone can help me with this problem.
We are trying to find a sensor detection to know how many persons or if any person was travelling inside a taxi.
We found the press sensors but they are quite expensive and makes the sollution not afordable for clients.
the RFID sollution I saw desribed cannot be applied as every passenger will be diferent each time.
So, i am trying to find something cheap and easy to install.
Any idea what kind of sensors could we work with¿
forget about cameras, volumetric sensors or any sollution the taxi driver could have to use...

Thanks in advance.