When looking for ticket, open up incognito mode in chrome browser and look up at skyscanner.net. The cheapest I found for your criteria was 1330 from turkish, although the trasit time was a bit more. the search as 4th sep -28 dec jfk-ktm rt.
Stay in incognito mode until you complete the purchase because websites like kayak store cookies on your computer to check if previous search occoured. 
Also here is a note from Zen ""In many cases, there is a long layover in Istanbul on the return from Kathmandu.  TK will provide a complimentary hotel if the layover is over 10 hours.  The hotel is outside the airport, so Nepali passport holders will need a Turkish visa to use these complimentary facilities.  For more information on the Turkish visa, please contact the Turkish Embassy website in Washington DC.  American passport holders can get an e-visa quite easily and details are available from https://www.evisa.gov.tr/en/"