A couple of thoughts.

The valve cores are essentially the same thing used in your tires. The old one just unscrews, and the new one screws in. When I went to get new ones for an a/c project, the shop just gave them to me; they aren't worth charging for.

Assuming the system is discharged, changing both cores requires, oh, approximately two minutes. No, wait, I'm being too pessimistic, make it one minute.

There are tools out there which can be used to change the valve cores without releasing the refrigerant. Do some browsing on http://www.acsource.com - I think they have an example there. Of course, with the recovery machine all shops now have, it's really no work to take the refrigerant out of your system, change the valve cores, vacuum the system, and then recharge it. That shouldn't take more than 30 minutes, with about 25 of those waiting for the vacuum pump to do its job.

Final thought. The valve cores aren't really expected to completely seal. A good set of caps for the service ports is really what keeps the refrigerant inside. The valve cores only have to be good enough to not let the refrigerant out between the time the charging hoses are disconnected and the caps are reinstalled.

How much do you trust this shop? 'Cause frankly it sounds a little like they're scamming you.

- JimY