THORChain and Fees

Understanding gas_rate
THORNode keeps track of current gas prices. Access these at the /inbound_addresses endpoint of the THORNode API. The response is an array of objects like this:
{
    "chain": "ETH",
    "pub_key": "thorpub1addwnpepqdlx0avvuax3x9skwcpvmvsvhdtnw6hr5a0398vkcvn9nk2ytpdx5cpp70n",
    "address": "0x74ce1c3556a6d864de82575b36c3d1fb9c303a80",
    "router": "0x3624525075b88B24ecc29CE226b0CEc1fFcB6976",
    "halted": false,
    "gas_rate": "210"
}
The gas_rate property can be used to estimate network fees for each chain the swap interacts with. For example, if the swap is BTC -> ETH the swap will incur fees on the bitcoin network and Ethereum network. 
The gas_rate property works differently on each chain "type" (e.g. EVM, UTXO, BFT).
Fee Calcuation by Chain 
THORChain (Native Rune)
The THORChain blockchain has a set 0.02 RUNE fee. This is set within the THORChain Constants by NativeTransactionFee. As THORChain is 1e8, 2000000 TOR = 0.02 RUNE
Binance Chain
THORChain uses the gas_rate as the flat Binance Chain transaction fee.
E.g. If the gas_rate = 11250 then fee is 0.0011250 BNB. 
UTXO Chains like Bitcoin
For UXTO chains link Bitcoin, gas_rateis denoted in Satoshis. The gas_rate is calculated by looking at the average previous block fee seen by the THORNodes.
All THORChain transactions use BECH32 so a standard block size of 250 bytes can be used. The standard UTXO fee is then gas_rate* 250.
EVM Chains like Ethereum
For EVM chains like Ethereum, gas_rateis denoted in GWai. The gas_rate is calculated by looking at the average previous block fee seen by the THORNodes
An Ether Tx fee is: gasRate * 10^9 (GWEI) * 35000 (units).
An ERC20 Tx is larger: gasRate * 10^9 (GWEI) * 70000 (units)
Inbound Fees
Interfaces set the inbound fee. Fastest should be used where possible for a better UX. 
Rune Code Example https://github.com/xchainjs/xchainjs-lib/tree/master/packages/xchain-thorchain#get-transfer-fees​
Bitcoin Code  Example: https://github.com/xchainjs/xchainjs-lib/tree/master/packages/xchain-bitcoin#get-fees--feerates-estimations​
Ethereum Code  Example: https://github.com/xchainjs/xchainjs-lib/tree/master/packages/xchain-ethereum#get-transfer-fees-estimations​
Always use a "fast" or "fastest" fee, if the transaction is not confirmed in time, it could be abandoned by the network. You should allow your users to cancel or re-try with higher fees.
Outbound Fees
3 * the inbound fee is set for Outbouds. This should be added up and displayed to the user as the total fee. For any transaction:
Total Fee = Inbound Fee + Outbound Fee 
See Outbound Fee for more information. 
Affiliate Fees
In the swap transaction you build for your users you can include an affiliate fee for your exchange (accepted in $RUNE or a synthetic asset, so you will need a $RUNE address).
The affiliate fee is in basis points (0-10,000) and will be deducted from the inbound swap amount from the user. 
If the inbound swap asset is a native THORChain asset ($RUNE or synth) the affiliate fee amount will be deducted directly from the transaction amount. 
If the inbound swap asset is on any other chain the network will submit a swap to $RUNE with the destination address as your affiliate fee address. 
SWAP:CHAIN.ASSET:DESTINATION:LIMIT:AFFILIATE:FEE
Read https://medium.com/thorchain/affiliate-fees-on-thorchain-17cbc176a11b for more information. 

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Outline

Understanding gas_rate

Fee Calcuation by Chain

THORChain (Native Rune)

Binance Chain

UTXO Chains like Bitcoin

EVM Chains like Ethereum

Inbound Fees

Outbound Fees

Affiliate Fees