I was fortunate in almost accidently becoming co-author of a computer game. There was no advance for this deal, but I received royalties quarterly for the number of units sold.
A big problem at the time was shelf space. This was before the advent of the internet. If you didn't get visibility on the shelf in the stores that sold computer games, you weren't going to get sales.
The game did fine for about a year or two but then tapered off quickly. An item almost unnoticed at first was the fact that the royalties were computed as "sales less returns". The last couple of quarterly statements I received showed that returns exceeded sales and so I owed the publisher money. Soon thereafter the quarterly reports ceased.
It was good while it lasted.
I am writing a book about a group of kid wizards who are looking for a ring. It also includes dragons and a wicked witch of the east. The 3 would be wizards are a tinman and a cowardly lion and a scarecrow.
There is also a little girl who was accidentally swept into the world and is trying to get back to her home in Kansas.
It might do well.....