Ahh, well defining "reasonable cost" is part of the issue. Is it reasonable to provide food to someone who you are currently at war with? How about technology? Is it reasonable for the U.N. to spend billions to build desalinization plants for a nation that spends all of its money on its military that it then uses to oppress its own people?
Its easy to say that it costs $2.00 USD to feed a person in a poor nation, but if we simply send $2.00 per person, per day, we have no assurances that the people will get the food. Even if we send the food, there are no assurances. In fact, in the past, it has been shown that many governments will appropriate money and food for their own "elite" class and allow the poor to continue starving. So the cost may be $2.00 in food, but there is also the organization needed to deliver the food to the appropriate people. This, of course, makes certain rulers very upset (because they don't get to steal all of the food or money), so they in turn refuse to allow the humanitarian aid in their countries. Now there's an even greater cost, the cost of removing that ruler (which may mean thousands of lives and hundreds of billions of dollars).
Yes, everyone should be fed. During the cold war, the United States gave (not sold) thousands of tons of food to the USSR during their famine times. This allowed the USSR to survive during times when they couldn't feed their own people with their own resources and didn't have the hard currency to buy food. The U.S. and other nations still ship food and water all around the world to help with starvation. There are different groups worldwide who are trying to feed people regardless of politics, national boundaries, etc. Unfortunately, there are simply some national leaders of the neediest nations that are more interested in gathering their own power than in feeding their people. Until those governments are changed, there simply cannot be a solution to the hunger in those nations.
To give you an example of this, Live Aid, which was formed to help the starving in Ethiopia, raised over $200 million USD for famine relief. Most of this money was taken by the government of Ethiopia and relatively few people in Ethiopia received any food from this charity. The corruption in Ethiopia raised the "reasonable cost" of feeding Ethiopians to an unreasonable level.

As to making mistakes, they are called mistakes because they are unintended consequences of actions (or inactions). If they were intended, they wouldn't be called mistakes, they would be called something else (atrocities for example). If I'm driving and my car skids on black ice and slides into a pedestrian, that is a mistake or an accident. If I drive directly into the person intentionally, that is murder.
To prevent all mistakes, we'd need to be perfect, and none of us are. To prevent intentional acts, we'd need to be able to predict the future. In 1936, nobody really suspected what Adolph Hitler's Third Reich might be capable of, but if they had stopped him at the Rheinland in '36, then millions of lives wouldn't have been lost during the European portion of WWII. Instead, politicans believed that Hitler could be appeased with the Rheinland and weren't prepared for his long term strategy. This is simply because of one fact. We can't tell the future. It is easy to say we should have known, but how could we have known? What bit of evidence available in 1936 would have told us?
A morally responsible person tries to do his best. That is all that can be expected. Sometimes we'll make mistakes, even some mistakes that cannot be taken back. There are unintended consequences to most actions. We can try to anticipate them, but sometimes they are completely unexpected. When the unexpected happens, we simply have to try to deal with it as best we can. The morally responsible act is not necessarily to avoid all mistakes (because that is impossible), it is instead to take responsiblity for one's mistakes and do what can be done to right them.
A morally responsible person may also try to right the wrongs of others (both intended and unintended) depending on the resources available to that person. If a morally responsible person is in a position of great power, it may be his responsibility to use that power to right wrongs on a large scale. This is one of the things the Presidents of the United States (past, present and future) are supposed to do. Of course there will always be those who object to whatever actions anyone may take because they may disagree as to what is right and wrong, and as to methods.

-- Jeff

Did you have a specific issue that you are dealing with or are you just trying to determine a personal philosophy?

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"Government is not the solution to our problem, government is the problem." --Ronald Reagan