<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by mwielsch:
(though JFK taxes are $80 if I'm not wrong so this is close to an extra segment)</font>
Nope. The rule of thumb for US taxes is $11 per extra flown segment: $4.50 XF to the airport (varies from 0.00 to 4.50), $4.00 ZP as segment tax, and $2.50 AY as security fee.
JFK "only" charges $3, and, assuming that you bought your RTW outside of the US, you don't have to pay the segment tax, so in your case you would be out a measley $5.50 by adding JFK to the trip.