tl;dr version: I suspect AA are treating LHR as a stopover for pricing purposes.


Searching on ITA for a biz one-way....

MAD-xDUS-LAX on IB then AB is the following on top of the base fare:
German Passenger Service Charge (RA) $20.30
AB YQ surcharge (YQ) $191.70
+ a bunch of spain and US taxes

MAD-xHEL-xLHR-LAX on AY then AA is the following on top of the base fare:
United Kingdom Passenger Service Charge (UB) $48.70
AA YR surcharge (YR) $206.40
Finland WL (WL) $5.70
Finland Air Transport Supervision Charge (XU) $1.40
+ the same bunch of spain and US taxes.

So adding in the $100 change fee and you'd expect about $150 to change.


Wondering how how much it would be with a stop on LHR...

MAD-xHEL-LHR on AY followed by LHR-LAX on AA with a stopover in LHR would be:
Finland WL (WL) $5.70
Finland Air Transport Supervision Charge (XU) $1.40
AY YR surcharge (YR) $54.10
+
United Kingdom Air Passengers Duty (GB) $218.10
United Kingdom Passenger Service Charge (UB) $64.60
AA YR surcharge (YR) $252.60
+ the same bunch of spain and US taxes.

Which would be more like $400 + $100 which is close to your $482.