Jay bo,
I'd say a 1000USD would be the lowest to bring-and even at that you'd want to cook at home-be careful with buying too much beer and try and secure a place to live as soon as you can.
Maun is very expensive, a rude awaking in fact. Going on a naive assumption that average income of the population was lower than Latin America, I believed that Botswana would be much less. Wrong.......
Bring as much cash as you can, if not, you could perhaps ask one of the backpacker lodge owners if you could stay at a discounted rate or tidy up in exchange for rent.
Of course, if you get on with Sefofane in Maun, they'll pay for your accomodation while waiting for the permit processing. In which case, you wouldn't have to bring a grand with you.