I'm sorry to break it to you, but Carson isn't running for President. He's using the donated money mostly on fundraising rather than build a campaign infrastructure. One of his fundraisers is an exec from a direct mail company which is siphoning off the money to send mail to gullible people. He's also squeezing in time for a book tour in the middle of a campaign. A similar scam is being perpetrated by a number of PACs in the conservative entertainment complex. They raise money apparently for conservative causes and spend most of it on salaries or paying a few companies that they have interests in.
Carson, Huckabee, Santorum, Jindal etc are using this campaign to raise cash, get some publicity and land some paid speaking assignments, book deals or TV shows. I'm not sure about Trump yet, he may have got into this as a joke but appears to be a bit more serious now.