I think Trek actors would be a lot more lucrative.
It depends on how many copies they think they would sell vs. the price commanded by a known voice to read the book. So, would having Frakes read 'Titan' increase the predicted sales sufficiently to pay the price of having Frakes read it? I don't know, but probably not (since he probably commands a pretty high price by now- Sirtis would probably be affordable. But then most audiobooks are read by men).