Sharkgirl - welcome to the forum.
The retailer etrailer.com has
a wealth of information on WDH options that you can review and get knowledge as well as competitive pricing. Installation of my Equal-i-zer (that is a brand name, not to be confused with "equalizer" which is a generic term) took about 1/2 hour when I got my new trailer so at, say, $120/hour chargeout rate, a deal should not charge much more than $60 to install. Add that to the price and you would see how much profit the RV dealer is trying to claw back from you.