Hi Dawn,
       Yes, the AC is worth having in the desert!  It should work much better
than it apparently does.  When I drove across the southern US (AZ, NM, TX,
LA, FL) I had it on the whole time and it was nice and cool.  Of course, last
week I broke a hose fitting on it, so maybe I am not the best source of info
today!
       First, if they are charging you that much they are probably using
r-12, and wanting to flush out the system.  R-12 works better than R-134, but
it is much more expensive and inherently evil to the earth.  See what they
charge to switch to R-134.  Either way, flushing the system probably isn't a
bad idea.  If you switch refrigerants, you should do this anyway.
       Next question, how much of a mechanic are you?  There are a few things
you can do yourself to save money, and make your system better.  If you are
so inclined, you can throw in a new expansion valve block and a
receiver/drier relatively cheaply, and swapping the seals for an R-134
switchover is easy.  Now, getting your system vacuumed and filled is a lot
cheaper.  (always pull a vacuum!)
       When you do get the system up to speed, you will find that it can be a
bit slow to cool off.  The volume of the van is huge, and the ductwork sits
on the ceiling so it starts out very hot.  But once it gets going it works
well, especially on the highway.  There is also a product out that has been
argued over on the list before, called ES-12.  It is supposed to help, but it
is definitely a DIY thing.

Best of luck

Bill Marshall
85 GL Tiico w/ retrofitted '90 A/C