Yes. It is expensive even for a single installation.. Now, if the cost were $50 to install a seatbelt, then the magnitude of the issue of "costs of installing seat belts" wouldnt be so big and instead people would be judging the "need for seat belts" based on merits. Does it really improve the net safety for children.
No, I dont think each seat belt would need to cost 150-250 bucks especially when these are installed in bulk contracts.