Off Exit 17 is probably your best bet (that i know of). Between the confluence of I-87 and I-84, and Stewart Airport, there are some options there. The drive to West Point from there is pretty easy, and kinda fun with the little jaunt over Storm King Mountain.
Hotwire. I use it for everything. I've stayed at 4 star hotels for less than $150/night and 3.5 star hotels for under $100. I stayed at the Andaz Wall Street for like $100. The only catch is you don't know which hotel you are going to get. Search by star rating and stick to the 90% recommended and above and I don't think you'll be disappointed.