This is a quote that the Lost Causers love to mangle (most notably the consummate Lost Cause mangler himself, Reverend Robert Dabney). But here's how Baldwin described that part of the conversation in sworn testimony to Congress shortly after the war:
[President Lincoln] said something about the withdrawal of the troops from Sumter on the ground of military necessity. Said I, "that will never do under heaven. You have been President a month to-day, and if you intended to hold that position you ought to have strengthened it, so as to make it impregnable. To hold it in the present condition of force there is an invitation to assault. Go upon higher ground than that. The better ground than that is to make a concession of an asserted right in the interest of peace."-""Well," said he, "what about the revenue? What would I do about the collection of duties?" Said I, "Sir, how much do you expect to collect in a year?"-Said he, "Fifty or sixty millions." "Why sir," said I, "four times sixty is two hundred and forty. Say $250,000,000 would be the revenue of your term of the presidency; what is that but a drop in the bucket compared with the cost of such a war as we are threatened with? Let it all go, if necessary; but I do not believe that it will be necessary, because I believe that you can settle it on the basis I suggest." He said something or other about feeding the troops at Sumter. I told him that would not do. Said I, "You know perfectly well that the people of Charleston have been feeding them already. That is not what they are at. They are asserting a right. They will feed the troops and fight them while they are feeding them. They are after the assertion of a right."
You can see the whole thing here:
http://valley.lib.virginia.edu/VoS/personalpapers/documents/augusta/p3baldwininterview.html#baldwin2
Note that Lincoln's talking about evacuating Fort Sumter here, not about letting the Southern states go. And he's asking what he would do about the collection of the revenue at the fort (which was a main function of the fort in the first place), not asking how he would pay for the government. But it also appears that even Baldwin's statement is wrong, because he quotes Lincoln as saying he expected to receive 50 to 60 million dollars in revenue annually. That's the TOTAL amount of ALL revenue collected at ALL U.S. ports - the vast majority of which was collected at Northern ports.
However, Baldwin's counter-argument is absolutely correct - the amount of revenue collected at ALL U.S. ports wouldn't have funded two weeks of civil war. Lincoln was very much aware of that. And you'll note that Lincoln doesn't argue about it and they move on to discuss other aspects of a proposed withdrawal, including feeding the troops.
It's all the Lost Cause crowd have to go on, but it isn't enough, so they have to mangle it and turn it into something that it's not.