A few questions about a water pump?
well first off you dont mention what size engine. if you have the 3.0 l v-6, then that price will be fair if they are replacing the timing belt and tensioners when they are replacing the water pump. as for a good place it depends on the mechanics and there certifications if they have any ase certified master tech. 20+ years exp.
does your van have a 3.0,3.3 or 3.8 litre engine? If it is a 3.0 liter engine then the job involves removing the timing belt. Book time is 4.5 hrs. The other two engine is much simpler, only 1.0 hr to replace the water pump. So since most garages charge about $100/hr it will cost you $450 and $100 respectively. Also it would be wise to change the thermostat and flush the system at the same time. That would be an extra 1.4 hrs or $140. Note: the serpentine belt has to be removed to do this job, so if it is worn replace it. There should be no extra labor, just parts cost.
It all depends on what size engine you have, the 3.0 is the worst and then the other two aren't as bad and 4 or 5 hundred seems like allot but these days when the garages are charging 100 an hour that probably isn't to bad specially in New York, the best place is probably the dealer.
$400 to $500 seems a bit high...should be closer to about $300.. .. call around to a few repair shops and get some prices...this is a pretty straightforward repair for an auto shop..