Yeah, well, the guy/guys in the shop all just love it when the owners come in the next day and say "So, I was on the internet last night...............", but what else can you do? And the tough part for anyone here is just the "approximately" 20% margin that they COULD have correctly diagnosed the problem.
Maybe you can just ask about how they know the EGR valve is bad, what the test steps were, just tell them you're concerned about spending $900 for it, etc etc. If they just say "well, that's what the code says it is" and never mention that they actually tested the valve or anything, then I'd be suspicious. Can you just pay whatever they say they have in it for diagnosis and walk away, or do you REALLY like these guys? The valve itself just usually isn't the problem, they need to attach a vac pump, like a mityvac, to the valve and open it and see if the engine runs like real crap or not. If it's opening (which they can just observe) and the engine pitch hardly changes, then it's a fair bet it's that tube, in which case my "personal" recommendation it to replace it with the updated one, but a "quickie fix" is to do what Steve B recommends in that link given above, which is to ream/clean it out with an old speedo cable on a drill. Look over that link by the way, it's one of the last items on that page. Maybe you could print that out and take it with you and show it to them (I don't really want them to read my "3 thoughts" I gave
). (PS maybe that means I'm a little shyer than you