Well, as it was said, p2p is the cheapest option and you wouldn't find any cheaper option rather than binance as it have the largest trade volume.

However, the withdrawal fees for BTC are pretty high and I've got a small lifehack for that: there is a platform called Nexo.io that grants at least 1 free withdrawal monthly (or more when they have promotion). Therefore, you can an altcoin on the cheapest network, exchange it to BTC and withdraw on the main net for free. I'm usually doing this when I need to withdraw small amount of ETH for transaction fees.


P.S. Nexo got a referral program that gives you 25$ in BTC if you hold 100$ there for a month. DM me for a link if anyone interested.