I have to agree with DarklyDreaming.

This appears to be a "studio" with no track record and no money. Unless they have proven otherwise this is just the same as any other hobby project you will find in the Help Wanted forum. The chance of an unfunded project even getting finished are minimal and even if it ships there is only a tiny chance that it will make any meaningful money. In short you are basically working for free.

Also, as others have said, 5% of what? Retail, wholesale, gross sales, net sales?