The total fare, although keep in mind that what you see on ITA is only an approximation. Specifically, it depends upon the fare rules for the outbound leg, but in most cases, you will keep the original fare and combine it, on a half-round-trip basis, with the historical price for the new one. (So, in this case, half of the round-trip PAR-LAX price plus half of the European-origin round-trip price for your new ticket — FRA-NYC or wherever you end up going. Most international fares are directional, and for a roundtrip fare, you look up both legs based upon the origin). The primary exception would be if your new flight doesn’t meet one of the combinability rules — e.g., it has a 30-day max stay and your new flight results in a 60-day stay. When that happens, the fare on the flown segment will be adjusted to the next-higher fare that can be combined. (There are also some situations where the return fare would be repriced using current fares instead of historical fares). So, there’s no way to get ITA to give you the exact price, because it’s not set up to price historical flights. But as long as the fare class is the same — e.g., if you booked a D fare for the original outbound, make sure that you’re still booking a D fare on your dummy reservation — I’d think they should be close.

The remainder, if any, will be available either as a flight credit (if it was a nonrefundable ticket originally) or as a refund to the original form of payment (if it was refundable).