Three of the current campaigns include a $2,700 option on their main donation page, according to the campaigns' "main" donation page (some campaigns have different donation landing pages depending on whether you've given before or other factors). That's the max amount an individual can legally give to a campaign in one election cycle. (Of course, thanks to the Citizens United court decision, deep-pocketed donors have any number of workarounds to pump money into the political system.)
You can think of that $2,700 donation as a decoy to make the other amounts seem palatable. A lot of restaurants have one item on their menu that's way more expensive than everything else, but they don't actually expect you to order a $50 steak. It's called the anchor, and it's there to make that $15 bowl of spaghetti look more reasonable.
Ben Carson, whose campaign is on the ropes, has the highest amounts listed on his default donations page. The preset options start at $25 and go to $5,400, which is the total amount a couple can donate. A spokesman for the Carson campaign explained that the totals were determined through a series of A/B testing.
"We adjusted these totals for a number of months but finally landed on this set being the most effective for the largest segment of our donor file," he wrote in an email.