I think this represents good value. We used Trailfinders for a similar trip last year and found them excellent. I did a comparison on their prices and found nothing untoward. Plus they give pretty good advice.
If you booked it yourself you would spend the following, assuming you are a couple sharing (all below in euro):
Flights (Delta) 710-750 (ref [broken link removed])
MGM 275 ea (4 nights.... ref www.mgmgrand.com)
New York Hotel* 300ea (3 nights in 4-star hotel... ref tripadvisor.com and hotels.com)
*lots of hotels in New York for 200 per room per night. The Jolly (ref hotels.com) is around 300 per night, so it a bit steep (which lets Trailfinders give the discount!!). Make sure you check out the Jolly on first to make sure it's getting good reviews!!
Total of 1285-1325ea => you might save up to 200 (100ea) if you booked yourself and went with a different hotel in NY. Sort of depends how fussy you are on the New York Jolly hotel; but it seems like an OK deal, but not great.
You could also try continental, a lady I work with goes to the states on a regular basis and swears by them , she recently went to vegas and florida, flights cost her 600 euro