The rack typicaly will last about 100,000 miles on most vehicles, the seals will start leaking, or inner gear will become loose making for sloppy stearing. The valve cover gasket will also start leaking, on many cars there are tubes tn the canter of the cover that house the sparkplugs, when they start leaking on these vehicles you will see a misfire. I would get another estimate, as the cost seams high, also dont tell the other shop what the dealership found, unless you trust them to be honest, many shops are good, but some will just comfirm what the dealer said and give you a slightly lower price.
It is common for the valve covers to be leaking, as for the rack it is not very common. I would recommend you get another opinion. We would give you an opinion for no charge and then you can compare prices if need be.