Why do Obama and Romney campaigns both set $3 as the entry-level donation?
Why do Obama and Romney campaigns both set $3 as the entry-level donation? - 
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David Brancaccio: If you are a residual Sex and the City fan, you may just have missed your best chance to hang with actress Sarah Jessica Parker. Last night was the deadline for entering to win a seat at that table. The cost of entering the sweepstakes: A $3 donation to Barack Obama's reelection campaign. Three bucks is also how much it cost to vie for a seat at George Clooney's recent Obama fundraiser, and Donald Trump's recent bash for Mitt Romney.

So what's with the $3, specifically?  Marketplace's Krissy Clark reports.

Krissy Clark: In most places, $3 won't even get you a gallon of gas. A cup of coffee, maybe. So why is three the number both campaigns have seized on in so many of their recent "asks"?

Here's Casey Barr's theory.

Casey Barr: You can barely buy anything with $3, so you're not going to miss it.

Barr should know. A few weeks ago, he got an email asking him to donate $3 for the chance to "grab a Bite with Mitt" -- and went for it. He's only 18, and had never donated to a campaign before. But heck, he'd just spent three bucks on:

Barr: An iced tea at Chick Fil-A. I said, 'why not'?

That's exactly the response campaigns are hoping for, says Adam Bonica. He teaches political science at Stanford and says lots of small donors look better than just a few big, rich ones. Plus?

Adam Bonica: The more people you get to donate to your campaign, even if you're not making a large profit off of them, the more people you have vested in your campaign.

Still, what's so special about $3? Why not $1, or $4? The Obama and Romney campaigns didn't return my calls, but The Center for Responsive Politic's Bob Biersack doesn't think the three is random. He says Obama's campaign in particular has mountains of data from the last election.

Bob Biersack: About how people responded to their requests for small contributions and maybe $3 looked magical somehow.

Of course, both campaigns will always take more. In fact, when you click on the link that says "donate," three bucks isn't actually an immediate option. The minimum donation for Obama is $15; Romney, $10. If you look hard, you can find a button for "other" amount to fill in the $3 your self.

I'm Krissy Clark for Marketplace.

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Follow Krissy Clark at @@kristianiaclark