You don't want to cab it to Baltimore from JFK! It's over 200 miles, very expensive and I doubt you'll find any local cab to take you there anyway. I believe you have three options:
1. Bus: take Airtrain from your terminal to Jamaica Station and E subway to 42nd St/Port Authority Bus Terminal ($7.25), or NYAS bus ($15) from JFK to Port Authority and take the bus to Baltimore from there. There are a few different bus lines with varying fares. Depending on traffic it's about 4 hours.
2. Train: Take the NYAS bus, E subway or LIRR to Penn Station and take Amtrak to Baltimore ($40+ depending on when you go). Three hours by train.
3. Air: there are flights from JFK to BWI-Baltimore airport on American and Delta.
JFK Airport Guide
You don't want to cab it to