The Force Bridge charges cross-chain fees in order to cover the operational cost of bridge infrastructure and making transactions on the Nervos and Ethereum networks. These fees, called bridge fees, are charged in-kind with the asset being transferred. Meaning when you transfer a token, a portion of the balance transferred goes towards covering the bridge fee.
Ethereum ($ETH) -> Nervos ($CKB):
The bridge fee is 400 $CKB worth of the asset being transferred. When you finish transferring from Ethereum to Nervos, you will get 400 $CKB along with the token you transferred every time. The $CKB you get can be used as transaction fees on Nervos Network, such as on Godwoken.
Nervos ($CKB) -> Ethereum ($ETH):
Nervos back to Ethereum, the bridge fee is largely based on the expected Ethereum transaction fee.
For now it is 0.015 ETH worth of the asset being transferred, which is 150000 gas * 100 Gwei gas price, and may be adjusted according to the average gas price.