Using 9/25 - 10/2 as a sample roundtrip, the $824 total fare breakdown at the cheapest economy class (O) that's not basic economy (B):

Fare 1: Carrier AA OLX7T1M5 LAX to LON (rules)
Passenger type ADT, ROUND-TRIP fare, booking code O
Covers LAX-LHR (Economy)
$75.50

Fare 2: Carrier AA OLW7T1M5 LON to LAX (rules)
Passenger type ADT, ROUND-TRIP fare, booking code O
Covers LHR-LAX (Economy)
$125.50


United States Passenger Civil Aviation Security Service Fee (AY) $5.60
Carrier-imposed surcharge (YR) $400.00
US International Departure Tax (US) $39.40
US Passenger Facility Charge (XF) $4.50
United States APHIS Passenger Fee Passengers (XA) $3.96
United Kingdom Passenger Service Charge Departures (UB) $64.50
United States Customs User Fee (YC) $6.11
United Kingdom Air Passenger Duty APD (GB) $96.60
United States Immigration User Fee (XY) $7.00
Subtotal per passenger $828.67
Number of passengers x1
Subtotal for 1 adult $828.67
Fare Construction (can be useful to travel agents)
LAX AA LON M 75.50OLX7T1M5 AA LAX M 125.50OLW7T1M5 NUC 201.00 END ROE 1.00 XT 6.11YC 7.00XY 3.96XA 39.40US 5.60AY 96.60GB 64.50UB 400.00YR 4.50XF LAX4.50

As you see, the outbound has a $75 base fare, with the return having $125 so if there was to be a credit given, you'd have to take into account that one leg may be priced lower than the other.

But also keep in mind in my example that LAX-LHR one way in O comes to $615 total, which is only about $200 less than the roundtrip.

Fare for 1 adult $615.80

This ticket is non-refundable.
Fare 1: Carrier AA OLX7T4M5 LAX to LON (rules) Passenger type ADT, ONE-WAY-ONLY fare, booking code O

Covers LAX-LHR (Economy)
$386.00
United States Passenger Civil Aviation Security Service Fee (AY) $5.60
Carrier-imposed surcharge (YR) $200.00
US International Departure Tax (US) $19.70
US Passenger Facility Charge (XF) $4.50
Subtotal per passenger $615.80
Number of passengers x1
Subtotal for 1 adult $615.80
Fare Construction (can be useful to travel agents)
LAX AA LON M 386.00OLX7T4M5 NUC 386.00 END ROE 1.00 XT 19.70US 5.60AY 200.00YR 4.50XF LAX4.50

If you're calling to change a trip in progress from RT to OW, they would likely reprice you based on the one way trip you took and price it accordingly. The truth is nobody here can tell you just how much you'll get as credit, if any. When you call, the customer service rep will have to kick it over the fare desk and put you on hold for sometime while they calculate it.