transmission was serviced by a mechanic and he said it will need to be rebuilt soon.

You'll need to call around your area transmission shops to get that information. Costs will vary between places.

Indeed, quoting this service is too difficult without knowing every bit of information. Also, the 5R55E transmission typically is not a great rebuild candidate. Failures are typically isolated to the valve body and main controls. Sure, anything can happen... A remanufactured assembly is probably a more cost effective repair in the event a major repair is indeed necessary.

Is the transmission experiencing any problems, or are you going off what this individual told you based on his experience? If you are not having any problems and the transmission is operating and functioning properly. Then I would not worry about it. Some people will say things with no regard to how it affects the owner of the vehicle. The truth be know, your trans may go forever of may fail in a month or two, but the same can be said for any vehicle. On a service it is normal to have some metal on the magnet of the transmission pan. I wouldn't worry about it unless you are having problems. If you want to replace it. I would also recommend a reman 3yr /100k mile warranty over a local transmission builder. But that is just my opinion.