Hello and welcome to Just Answer. My name is Denny; now let me see if I can help you with your problem.
Ball joints are nothing to take lightly. If they come apart, the whole van will become un-drivable, and may cause an accident, if you are driving at speed, when they come apart.
You could always go to a different Dealer, and get a second opinion.
You could try a local parts store, but some suspension parts are that way. I work primarily on Dodges, and there are some suspension parts that are only factory available.
One thing you could do is search for the lower control arm; sometimes the factory book will call it a ball joint, but it is actually the entire control arm. ( that may be why the cost is so high)