It should be noted that the reason authors get smaller royalties on tie-in books than on original novels is because the publisher also has to pay royalties to the licensor, which means any royalties get split between the author and the copyright holder. Same slice of the pie, but cut into smaller slices to serve more people.
For example: Let's say a publisher is offering a 8% percent royalty on paperback copies. On an original novel, the author would get the whole 8% (provided the book earned out), but on a tie-in novel that 8% would be divided between the author and the licensor--with the licensor typically getting the lion's share because it's their characters and universe after all. So the writer might get 2% and the licensor would get 6%, adding up to the same 8%.
I had no idea that, with a tie-in piece of work, the compensation would be so low. If I was on a game show and had to guess, my figure would have been much higher. 2% hardly seems like anything at all when you consider you are probably spending months and months working on a novel.
^Well, the rationale is that the licensors are the ones who actually own the property, and the royalties are how they make a profit from licensing it. From a legal standpoint, they're the "author" of the work, and we're their subcontractors. So they get the lion's share of the royalties and we get a cut as compensation for our efforts.
It's actually a lot harder to earn out an advance on a tie-in, because the royalty percentage is so low. But the compensation is that the advances are relatively large.
They would have to be to make it worth your while, I would think. Otherwise, you would probably need to find other work in order to make a decent living. I certainly hope that the print runs are high.