I'm not sure if I'm understanding what the question is here but let me try and give you my two cents.
If you are at the dealership and the mechanic is telling you it's not worth it, I would find a different mechanic. Car dealerships typically incentivize their mechanics to talk existing customers into purchasing new vehicles over repairing the vehicle they have.
With that being said, if you love this vehicle, I'd use this site and find out what the fair market value of your vehicle is. Consumer reports recommends spending no more money on repairs then the value of your current vehicle.
An example, if the current value on your vehicle is $4000 and you can repair it for equal or less than that they recommend going with the repair. They back this position up by explaining spending $4000 on a vehicle that you know, is better than going out and buying another vehicle that may have similar or even worse problems than the vehicle you have. In short you know what your current vehicle is like and if it has not been nickel and diming you to death then a major repair may be in your best interest.
I would look for a local independent automatic transmission specialist, take your vehicle to them and get an estimate. Then do some research on the better business bureau to make sure they are a reputable shop. Independent shops typically service their customers better as opposed to bringing it back to the dealer.
I hope this helps,