Last year my local dealer quoted me £45 for half an hour, to include any re-setting (eg airbag light) that they might be able to do within the time. Maybe you could persuade them that half an hour should be enough to 'get the error codes'.... it should be, shouldn't it?
BTW when I had a BMW and wanted to diagnose what I thought was a cam sensor (turned out to be crankshaft sensor) , they wouldn't quote me a price, just an hourly rate! I said I just wanted them to plug the thing in and tell me what error codes appeared. They said "it doesn't work like that - we find an error code, fix that problem, then look for the next error code, fix that, etc, etc, so there's no way we can give you a quote"
. Needless to say I didn't go down that route, but fortunately found an independent specialist who charged me a fixed fee (about £50 in today's money), charged me list price for the sensor and fitted it for free. Now that was good value.