Wombat on Grand Street btw. Leonard and Lorimer has lobster night every Tuesday. Lobster (no claws - boo) choice of 3 different preparations for $12. I have yet to try it myself, so I am not able to truly say if it is a deal or not.
the best lobster deal in town is The Palm - unfortunately, it only exists from June 1 thru August 5th. 5 lbs lobster for 2, includes 2 salads & 2 sides for $105. 6 lb lobster for $125
if you can get past the dated decor try francisco's centro vasco on w 23rd st between 6th & 7th aves. not trendy and not a hispter joint and no lobster rolls.
i'll second centro vasco, though you have to get the lobster broiled, it's my favorite. as far as this being the 'best deal'...well that depends on how much lobster you eat. i usually do a 3 1/2 pounder that's around $60-70.