Basically the fare from EZE to MAD allows you to stopover at EWR. But the transatlantic portions requires you to depart and arrive into JFK and not EWR. The fare you got has no stopovers. By stopping over, you incurred a $50 stopover fee actually for the London stopover. But by stretching the fare to go via London, you exceeded the mileage by 5M, and then there're fuel surcharges on top of that.
The lowest I can come up with a B class fare is US$1942. Flying from EZE to EWR, but from JFK to LHR and back... you must fly into JFK. Flying into EWR just changes the fare entirely.
Yes the fare to MAD is cheaper to LHR. And it permits you to stopover in LHR.. so why not just do a MR into MAD and back...
This is not exact to your specific flights but it at least prices out:
1 AA 908B 19SEP M*EZEMIA SS1 955P 543A 20SEP T /DCAA /E
2 AA1970B 20SEP T MIAEWR*SS1 829A 1121A /DCAA /E
3 AA 100B 03OCT M JFKLHR SS1 630P 625A 04OCT T /DCAA /E
4 IB3163B 10OCT M LHRMAD SS1 1055A 215P /DCIB /E
5 IB3172B 10OCT M MADLHR SS1 325P 445P /DCIB /E
6 AA 107B 10OCT M LHRJFK SS1 630P 905P /DCAA /E
7 AA1999B 08MAR W*EWRMIA SS1 613P 924P /DCAA /E
8 AA 943B 08MAR W MIAEZE*SS1 1115P 954A 09MAR Q /DCAA /E
BASE FARE......................TAXES.............TOTAL
1- USD1942.00....................289.80XT......... USD2231.80ADT
XT 6.40ZP 56.40US 10.00YC 14.00XY
9.90XA 10.00AY 97.10AR 2.50TQ
10.00QO 9.00GB 42.80UB 5.00RS
1.70QV 15.00XF
1942.00 289.80 2231.80TTL
ADT-01 BLXE6M
BUE AA X/T/MIA AA T/EWR //T/NYC AA LON Q55.00 IB X/MAD 5M
BUELON1000.12BLXE6M IB X/LON Q55.00 AA T/NYC //T/EWR AA X/T
/MIA AA BUE 5M725.02BLXE6M 1S50.00 P BUELON56.86NUC1942.00
END ROE1.00 ZPMIAEWR XFMIA4.5JFK3EWR3MIA4.5
If you are to fly DIRECT with no stopovers no transfers within the US etc.. and fly the most direct route, you will get your $1491 fare.
1 AA 908B 19SEP M*EZEMIA SS1 955P 543A 20SEP T /DCAA /E
2 AA 68B 20SEP T MIAMAD*SS1 615P 855A 21SEP W /DCAA /E
3 AA 69B 03OCT M*MADMIA SS1 1145A 315P /DCAA /E
4 AA 909B 03OCT M MIAEZE*SS1 840P 632A 04OCT T /DCAA /E
.......BASE FARE..................... TAXES......... TOTAL
1- ...USD1491.00...................172.80XT...... USD1663.80ADT
XT 28.20US 10.00YC 14.00XY 9.90XA
5.00AY 74.60AR 2.50TQ 10.00QO
7.90RS 1.70QV 9.00XF
1491.00 172.80 1663.80TTL
ADT-01 BLXE6M
BUE AA X/T/MIA AA MAD Q55.00 M690.50BLXE6M AA X/T/MIA Q55.00
AA BUE M690.50BLXE6M NUC1491.00END ROE1.00 XFMIA4.5MIA4.5