That's a normal charge these days.
Can you get the codes read and printed out? Obviously don't clear them, that way you'll know if they are at it or nor.
I know some dealers here are more than that! We had a customer call the dealer to ask if something would be covered under warranty, the answer was to book it in to get it looked at but obviously you'll be charged for diagnostics. The customer explained she just needs to know if the part would be covered or not, all they wanted was the diagnostic fee!
Recently had someone else get warranty work done at dealers, all covered but they gave them the diagnostic fee at the end!! Luckily they were switched on and stood their ground so got it withdrawn.