And I found that choosing hotel and flight separately was significantly cheaper for me than buying a package.
I finally decided on the Park Central Hotel, which unsurprisingly is close to the south side of Central Park, but also very close to Times Square and Madison Square Garden. It's a **** hotel, rooms look alright, guest feedbacks are very good, I'll find out!
Actually the best offer I found was directly via tripadvisor - a 40 % winter discount for booking on the hotel's own online reservation system. That took the daily room rate for a superior room considerably below 100 Euro, but with all the applicable taxes a little above 100 Euro ;-). Still, very good price!
Having read everywhere that rooms in New York hotels usually are tiny I was really surprised to find spacious junior suites - for example at the Shoreham (near Empire State building) - or even huge flats with living room, kitchenette and balcony (!) at affordable prices at the Marmara (Upper East Side). But I wanted to be more in midtown and close to Central Park.