Security procedures cost the airline and the Israeli Govt a very great deal of money, but I don't think they have to pay for the special protection on the airports. I would think it is the same as for the security around El-Al's offices.
Airliners have been targets for terrorists, and that's why there are long (more or less effective) security checks before boarding. And a civil airplane is also an easy target while on the tarmac. There have been deadly attacks against El-Al planes on the ground in Athens (1968) and in Zürich (1969), and an attempt in 1975 in Paris. So that's the reason for the armoured escort. Sometimes, it is a military vehicle doing the job (I remember seeing them in GVA and ZRH).