Voting during an election has an opportunity cost associated with it. Elections are not held on holidays and a person who decides to vote usually has to spend a lot of time waiting in line for her/his turn to come. People in low-income groups are usually paid by the hour and the time spent waiting to cast a vote comes with a loss of income as the person does not receive the wages that would have been earned if the same time was spent at work. Voting is in a way not economically feasible for those in low-income groups. This is also a reason why elected representatives are more inclined to look at the needs of those in the higher income groups before the needs of those in the lower income groups are considered. This provides another reason why people in lower income groups vote less and a circle of sorts is created.
On the other hand, people in the higher income groups can claim that it is their responsibility to vote as they are part of a democracy because as they can afford to do that. The loss of a day's wages does not impact their finances as much as it does to someone in a low income group. Their claim that they "vote for reasons that are borne out of a sense of duty rather than from economic interests," could be true as their financial state allows them to do so.
Here in California, the opportunity cost of voting is a first-class postage stamp on the envelope of an absentee ballot.