In terms of transport, a bus from São Paulo to Foz will cost about R$125. To go to Bonito by bus from Foz you need to change buses in Dourados. The total cost of the trip from Foz to Bonito should be about R$150. www.buscaonibus.com.br is a useful website for finding out bus timetables and prices, but unfortunately doesn´t work in the Bonito area as the only bus company which serves Bonito (Cruzeiro do Sul) is not listed on the site.
I´m afraid I don´t have recent prices for visiting the falls in Foz.
I suppose you are aware that tours in Bonito are generally considered to be quite expensive? There are a few activities which you can do on a budget, which are well worth it, however if your budget is really tight you might want to check out the tour prices before you make the trip all the way there.