NYC Marathon is possibly the biggest attraction week/weekend of visitors to NYC - hotels charge a premium. Maybe look at locations outside of Manhattan to reduce your cost but it does cost to do marathon:
Entrance fee $185
Airfare $400+
Hotel $300-400 p/nt
Any extra souvenirs (100+)
And that qualifies as the most costly form of transportation in the world to run 26.2 miles or 42 km to receive a metal and weak/sore legs for a few days. HAHA!! I do them too so maybe I want company in the misery of cost.

The next big NYC event will be the NewYearsEve and SuperBowl.