Generall for midweek travel (Depart Tue/Wed/Thur) from November right through to the end of service; requires Saturday night stay.
For PRG, generally require 2 connections (RDU and JFK) or a change of airport in NYC. Price drops to as low as $1698 in May, but is available for a little more ($1728 and upwards) from November onwards.
PRG example
For BRU, the direct DL flight is available, starting price is $1998. Z class (see below)
other examples:
ZRH -
from $1938
LUX - from $2038 (via CDG, and including CDG-LUX-CDG on Luxair)
MAD -
from $2137 on direct, non-stop DL services
BCN - from $2192
Venice/Bologna/Florence/Milan - from $2277
DUB - from $2288 (nonstop direct DL services)
LIS -
from $2229, via FCO (Rome) and including TAP segments FCO-LIS-FCO;
ATH - from $2347