A dollar feels more important than it actually is, so people treat the bets seriously even though they are not very serious.
Although there is a weight in the dollar, I think there is also another reason why people take it more seriously. People adjust their believe according to what other people believe and their confidence level. Therefore, when you propose a bet, even only for a dollar, you are showing a high confidence level and this decrease their confidence level. As a result, system 2 kicks in and they will be > [forced] to evaluate honestly.
Although there is a weight in the dollar, I think there is also another reason why people take it more seriously. People adjust their believe according to what other people believe and their confidence level. Therefore, when you propose a bet, even only for a dollar, you are showing a high confidence level and this decrease their confidence level. As a result, system 2 kicks in and they will be > [forced] to evaluate honestly.