Going to Cuiaba, fly to Rio or Sao Paulo?

Hi,

I'm planning my Brazil/Bolivia trip for next year. The plan is to fly to brazil, then take a domestic flight to Cuiaba, and from there to Bolivia (Noel Kempff Mercado), go back to Brazil, do the Pantanal, Alta Floresta, etc, then fly back the domestic flight, do a couple of days at some beaches, etc.

Naturally, my original plan was to fly to Rio, then Cuiaba, then going back, take a bus to Angra Dos Reis and then Ilha Grande, then back to Rio.

The Roundtrip flight Rio<->Cuiaba figures out roughly 450-500USD (R850-R900) at all websites (Expedia, decolar.com, gol, tam).

Flying from Sao Paulo drops the price to R250 (~140USD) on special promos (midnight flights) on tam/gol and via the decolar.com website (Expedia shows USD 300).

I'm now considering ditching Rio, flying to Sao Paulo instead (this saves almost 800USD for us two), and going for Ilha-Bela instead of Ilha Grande, skipping Rio altogether.

I know there is the Rio<->Cuiaba bus option, but at around 36 hours each way (correct me if I'm wrong), for a 3-4 weeks vacation, it is just too much time wasted, better spent elsewhere.

Now to the question part - Does anybody know a way to get cheaper flights going Rio<->Cuiaba, or is it 800USD versus a visit to Rio? (sounds too much to pay just to visit Rio)

Thanks,

Elad.