s&box does not charge to use the engine, similar to godot. But steam regardless of the engine you use changes you a 100$ recoupable fee to publish to steam. This is something every engine has to deal with. its not related to s&box and a valve thing. https://store.steampowered.com/sub/163632/
In order to get fully set up, you will need to a pay a fee for each product you wish sell on Steam. You can pay this fee with any payment method supported by Steam in your region, except methods that use the Steam Wallet.
This fee is not refundable, but will be recoupable in the payment made after your product has at least $1,000 USD Adjusted Gross Revenue for Steam Store and in-app purchases.