I used to travel from Fordham to New Roc daily, that platform was always packed in the morning with reverse-commuters for places like White Plains, New Rochelle and Greenwich. More than doubling the price to New Rochelle from $3.00 to $7.75 seems like a surefire way to kill New Haven Line travel from the Bronx to those lower stations (MVE-New Roc). Greenwich and Port Chester going from $5 to $7.75 might be easier to swallow, but it's still pretty significant. I'm not super familiar with the Harlem line, but Fordham-White Plains jumping in price seems like a similar disaster.
Bronx fares being affordable is probably a large reason why Fordham is such a popular station. Sure, the trains can get pretty crowded (at least, they did some time ago), but standing for 10 minutes until the train gets to Mount Vernon really isn't that bad. I don't see much of a point in shooing these riders to slower forms of transportation.