Yannis was technically right, but his answer doesn’t broach American culture, and culture is the framework for humor; hence why the Frenchman posed the question.
$27 is a popular punchline for the Bern (google “Bern your enthusiasm”) as it’s a metaphor for his grassroots campaign strategy, the most successful in history. When Clinton claimed on Twitter that she was “grassroots” with a million small donors, Sanders counter-tweeted that he had two million donors at an average of $27. Thus, it was a neutral joke for Obama to make, and relevant in its reflection on Bernie's political philosophy.
This was a focal point of Bernie's campaign as it highlighted his opposition to “Big Money", i.e. the abuses on Wall Street and the aftermath of Citizens United - a 2010 Supreme Court decision that justified the existence of Super PACs, which allow companies, unions, or individuals to indirectly make unlimited, anonymous donations to political campaigns. It is an incredibly controversial ruling, one that Obama and Sanders both strongly oppose, as it effectively silences the voice of the common people who cannot financially compete with modern corporations, on the basis that companies and unions are individuals, and that individuals vote with their dollar as a modality of the 1st amendment right to Freedom of Speech.
Bernie was the only candidate who did not have, or want, the support of Super PACs.