Timbo is a lovely hostel, though not the cheapest around. If that's your main concern though, there's a whole range of bland sleeping factories on offer as well with prices as low as 10 pesos a night (low season, back in 2006. Might have changed).
Also second Marlis: one day is not enough, although it is sufficient to see the main falls, walk up and down the boards, stand in line for your 'unique' picture of devil's throat and see the 'wildlife' (watch our for your belongings; both pickpocketers and animals will try to steal them). It's much better imo to take 2, maybe 3 days, walk to falls instead of taking a train, and maybe visit some of the more remote trails where the park is MUCH quieter.
[ Edit: Edited on May 30, 2008, at 12:27 PM by bentivogli ]