There are two things to consider here.
First, higher income people may well have more flexible work and life situations. It is often the case that they can get off of work for a bit to go vote where a lower income person working, for example, in a retail outlet, would not be able to. This means that the overall cost of voting would be lower for the higher income person even if their time is more valuable.
Second, voting decisions are made based both on costs and on benefits. It is likely that higher income people derive greater benefits from voting. These are psychological benefits, of course. For higher income people, the system seems to work. They do not typically feel that government has abandoned them. Therefore, voting can be more satisfying to them because they feel as if they are participating in an important civic event, one that works for people like them. Therefore, the benefit they get from voting is higher than that derived by a poorer person. The benefit is more likely to be sufficient to outweigh the costs of voting.