KLM has New York JFK at $467 round trip and Los Angeles or San Francisco at $526 to Belfast, Northern Ireland BHD city airport for departures beginning late August and available through March 2017. Chicago is $548.
Google Flights shows higher prices for many KLM flights which price lower when you click through fare to KLM site. Higher Delta prices shown in Google Flights at $500 from JFK and $560 for LAX/SFO appear to price accurately to DL site. Similar fares might be available from other KLM serviced airports in USA.
Many of these fares allow an overnight stay in Amsterdam or you can route differently to avoid the overnight, but that tends to raise price $20 to $40 for ticket routings I checked.
KLM Los Angeles LAX – Amsterdam AMS – Belfast BHD – Amsterdam AMS (overnight) – Los Angeles LAX
$525.77 Sunday Sep 4 – Tue Sep 13
KLM New York JFK – Amsterdam AMS – Belfast BHD – Amsterdam AMS (overnight) – New York JFK
$466.77 Sunday Sep 4 – Tue Sep 13
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