transmission 2000 chev impala

started last Thursday after the car was warm,whining sound coming from the transmission and shifting hard. Transmission shop says after doing
road test and plugging their scope into it that a couple of different codes are showing up (didn't tell me which ones) but what it boils down to is the computer is putting the transmission into high pressure limp home mode. They won't know why until they tear down the transmission $300cdn to pull it out and tear it down and all parts and labor on top of that. In your experience, what does this usually end up costing. They wouldn't even give me a ball park and once the tranny's ripped apart it's too late.
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