ERC-20 vs ETH – Does ERC-20 Token Transfer Cost More Gas Than ETH Transfer?

The EVM works with 256bit/32byte words (debatable design decision). Every operation is based on these base units. If your data is smaller, further operations are needed to downscale from 256 bits to 8 bits, hence why you see increased costs.

Btw, if you toggle the "Details" on the online solidity compiler, it will give you the exact assembly dump where the extra opcodes are from. I didn't have time now to interpret them, but if you do and find there's something extra, I'm sure the Solidity team would be happy to add optimizations to work around them.

You have to multiply the gasUsed for the gasPrice. The gasUsed is contained into the transaction receipt. While the gasPrice is into the transaction itself. Indeed, in your case, you're trying to multiply the gasPrice for the gas, which is the one provided by the sender (which can be higher that the gasUsed). ref: getTransaction, getTransactionReceipt

So, this is how your code should look like to be correct

const hash = await contract.buy.sendTransaction({from: buyer, value: amount});

// BALANCE AFTER TX
const balanceAfter = web3.eth.getBalance(buyer);
const tx = await web3.eth.getTransaction(hash);
const receipt = await web3.eth.getTransactionReceipt(hash);
const gasCost = tx.gasPrice.mul(receipt.gasUsed);

or you can also actually save the getTransactionReceipt call in case you call your method directly (in this case should give you a result that contains also the transaction receipt ref)

const txInfo = await contract.buy({from: buyer, value: amount});

// BALANCE AFTER TX
const balanceAfter = web3.eth.getBalance(buyer);
const tx = await web3.eth.getTransaction(txInfo.tx);
const gasCost = tx.gasPrice.mul(txInfo.receipt.gasUsed);