As Humberto stated, it should be easy; but depending on the complexity of the design, and how well it is implemented in the application, it could be longer: the worse the implementation (more probable for lower quality apps), the longer it will take.
Therefore, if the app itself is well-designed and follows best practices, the cost should be relatively low at about an hour or two.
However, this does not take into account ancillary work needed to re-publish the new app, which itself could take an hour, two, or more, depending on the issues that may arise.
if minor branding changes mean = changing the logo, putting new labels then its not a job of more than couple of hours (or even lesser than that).
If minor branding = logo change, change of whole skin, layout then it can be a little longer job.
It also depends upon what kind of contract you have with the App developer, like if they cover this "white labeling part" already in the cost or its a new demand from your side.