$900 to replace the EGR valve on a 104? Yeah, i have a few thoughts.
#1 thought: I wonder if they will ask you to squeal like a pig.
#2 thought: It probably isn't the problem
#3 thought: Where else can you take this car.
$900 is totally outrageous, it's less than 1 hour of work, even if you have problems, which sometimes does happen if the pipes won't break free. Lets say $200 worse case scenario, but that's if they notify you there are stuck pipes, they should be quoting about $100 labor. How much is the valve itself?
PS (again) You can go out to the car, raise the hood, and SEE the EGR valve. There are 2 bolts holding it down, and a pipe attached to each side, and 1 vacuum hose. NOT THAT TOUGH.