Unfortunately from Bergamo to Mantova there are nearly only regional trains and this is a very short distance!
So, if market prices are applied to long distance train tickets (Frecciarossa, Frecciargento, Intercity e.g.), for local and commuter trains there are Km-prices, quite low.
However, enjoy your trip, both cities are historical and very charming: don't miss "Palazzo Te" in Mantova!