Rent a car in Bahrain or Saudi if working in KSA & living in Bahrain?

Wherever you rent, you will need to get an NOC from your car company to be able to take the car across the border plus GCC insurance (vs. normal same country one).  This will be part of the rental rates and will cause them to increase.  The top up is only for 3rd party on the causeway and it is a standard rate which is lower if you buy for long term.

Thanks. In the past, I rented a car in Khobar for 2,200 SAR per month (3 year old Toyota Corolla), with an additional 100 SAR for the paper to cross to Bahrain as and when I needed. This paper was good for three months (or until my multi exit/re-entry visa or Iqama expired if sooner). Going across to Bahrain for a day or weekend, I simply bought the mandatory extra insurance at the causeway (3rd party only?). However, if I am going to live there, then I think I would feel more comfortable having fully comprehensive insurance in Bahrain, if renting a Saudi car, or vice versa if renting a Bahrain car. Hence the question about rates.

Generally speaking, the GCC insurance component costs a bit more depending on the value of the car and the rates passed onto you by the lease company.  For private vehicles, expect to pay 15-20% more on top of your insurance premium as a rough estimate.