There is no fee to open and use an Amazon Developer account. Additionally, there is no cost to submit an app to the Amazon Appstore.
The Small Business Accelerator Program allows developers that earn less than 1 million USD in total annual revenue in the previous calendar year to receive an 80% royalty share for each app or IAP sale.
If your revenue exceeds 1 million USD in the current calendar year, you will revert to the standard royalty rate. Please see the Amazon Developer Services Agreement for further details.