Not having your tiers in front of me, I can’t say definitively, but here’s what it sounds like.
Based on your tiers, access to your game is $20. You have subsequent tiers that offer other things (t-shirt, posters, sculpting, etc.). Those rewards aren’t royalty-bearing. So unless later tiers offer additional in-game benefits, the only royalty-bearing amount is $20.
So 5% of $20 for the first tier of $20, but still only 5% of $20 for the $500 tier.