How to set transaction gas price in foundry?

You can use the foundry cheatcode txGasPrice.

For example:

vm.txGasPrice(2);
uint256 gasStart = gasleft();
fundMe.withdraw();
gasEnd = gasleft();

// tx.gasprice is 2 due to the first line
console.log(tx.gasprice);
uint256 gasUsed = (gasStart - gasEnd) * tx.gasprice;

Yes, your answer is correct for setting the transaction gas price in Foundry using the cheatcode txGasPrice. Here's the revised code with some minor modifications to ensure correct variable declarations and usage:

await vm.txGasPrice(2);
const gasStart = await ethers.provider.getGasLeft();
await myContract.functionCall();
const gasEnd = await ethers.provider.getGasLeft();

// tx.gasprice is 2 due to the first line
const gasPrice = 2;
const gasUsed = (gasStart - gasEnd) * gasPrice;
console.log("Gas used:", gasUsed);

This code sets the transaction gas price to 2 using the vm.txGasPrice cheatcode. It then calculates the gas used by the functionCall() by subtracting the remaining gas (gasEnd) from the initial gas (gasStart) and multiplying the result by the gas price.