If I remember correctly entrance to the park itself was about $30. You obviously have to get there which costs less than that and you need a boat across the lake when you arrive which isn't much either.
I budgeted $60 a day when I was there in 07/08 and I don't think I went over that when treking in the TDP. There is ZERO need for guides. The paths are some of the clearest and well maintained I have ever been on anywhere.
I'd suggest reading people's blogs on it if you can find them to get some more insights.
Certainly do not pay the big bucks for someone to point you in the right direction.
If you stay in one of the refugio's they will can provide a meal in the evening and make a packed lunch for you (at a cost).
I found the TDP one of the hardest things to get decent info on whilst in SA. I imagine this means that companies can then charge a large premium to take you round. But then I am cynical at times!!