contracts/smart-contract-wallet/SmartAccount.sol:
  192:     function execTransaction(
  193:         Transaction memory _tx,
  194:         uint256 batchId,
  195:         FeeRefund memory refundInfo,
  196:         bytes memory signatures
  197:     ) public payable virtual override returns (bool success) {
  198:         // initial gas = 21k + non_zero_bytes * 16 + zero_bytes * 4
  199:         //            ~= 21k + calldata.length * [1/3 * 16 + 2/3 * 4]
  200:         uint256 startGas = gasleft() + 21000 + msg.data.length * 8;
  201:         //console.log("init %s", 21000 + msg.data.length * 8);
  202:         bytes32 txHash;
  203:         // Use scope here to limit variable lifetime and prevent `stack too deep` errors
  204:         {
  205:             bytes memory txHashData =
  206:                 encodeTransactionData(
  207:                     // Transaction info
  208:                     _tx,
  209:                     // Payment info
  210:                     refundInfo,
  211:                     // Signature info
  212:                     nonces[batchId]
  213:                 );
  214:             // Increase nonce and execute transaction.
  215:             // Default space aka batchId is 0
  216:             nonces[batchId]++;
  217:             txHash = keccak256(txHashData);
  218:             checkSignatures(txHash, txHashData, signatures);
  219:         }
uint256 txcost = 21_000;

 function changeTxCost(uint256 _amount) public onlyOwner {
        txcost = _amount;
    }
