How much does it cost for a new transmission for a 2015 Nissan Sentra? Is it cheaper to get transmission rebuilt?

I would highly suggest an evaluation by a local Nissan dealership near you. They can see if any warranty applies to your current vehicle. Most powertrain warranty policies last for 5 years or 60,000 miles, whichever come first. If you are under 60,000 miles, you are still under powertrain warranty. Besides this, your concern may not be transmission related at all. Seek a true and reasonable diagnostic test to find out what is going on. Like I said, in this case, it would be an advantage to seek guidance from your local Nissan dealership.