Jenn,
From the port authority's website regarding airport transfer options: http://www.panynj.gov/aviation/connframewr.htm
I don't remember a cab costing as much as number 6 states. I believe the rate is more in line with what is stated by the port authority.
I also think you could get a town car from EWR to JFK for less than $150 through either carmel or tel aviv:
http://www.carmellimo.com/
http://www.telavivlimo.com/
Your results with these and the other car services may vary.