Its not easy to say, different things will cost different amounts. But clearly don't ever pull out a wod of cash, cause they'll take you for whatever they can. You need to have little amounts available, and never let them see where the money is coming from. Usually, and of course this is all generalisations, have around 50-100 US available per day, especially travel days or days when you go through government agencies, basically if you need a service rendered by a governent agency in any way, you will have to pay bribes ie. permits etc. . You might also need to convert into their currency, or they may take a lot less cause its in US Dollars, just play it by ear. You get hit more on the roads and in cities than say out in the bush. Also its not aggressive, its just business, so you can barter. See what he asks for and offer him half, don't be rude but don't be a push over either, play it like you know the drill and none of this fazes you. Hopefully you won't have to bribe anyone and that extra money can be put back into budget...
Good luck man, you'll be fine and before long it will seem ordinary... The only country in Africa where you don't have to do business like that is South Africa, otherwise thats how it goes - in my experience anyway...
Do you have a local liason in each country?