Hi there! This sounds like a fun project. Normally, I would ask for a slightly higher fee for 5 finished minutes of recording, but since you say this is a test exercise, it sounds like you would be open to the consideration of paying more for future work if you were really happy with the test exercise? Normally, the lowest I would justify doing 5 finished minutes for is $40, with the prospect that there would be additional projects.
I have done a fair amount of work with character voices in audiobooks and animated characters. Some of my voice samples are available in my "Portfolio" section of my "Profile" page, but I will include a link here as well with examples that I think are similar to what you are probably looking for. The demos I would suggest playing are "Super Zero" and "Accent Sample Mix."
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