It's a tough transfer during rush hour -- 90 minutes is best case even by taxi. Cost is over $80 with tolls, or about $100 with tip. There is an airporter bus which goes directly to JFK (no changing buses) and costs $22 per person, also takes 90 minutes (buses and taxis are often within a few minutes in terms of travel time). You'll make it if flight arrives in EWR on time, but an hour's delay is pretty normal in EWR for 7:30 am arrivals (or so it seems).

EWR airport has an excellent web site, with lots of transportation options http://www.panynj.gov/aviation/connframewr.htm