(1) P2P, preferably your friend. You can hand him your private key, and he can give you his private key (barter). However, there will be problems if one side or both store backups. It's the cheapest, but not feasible in most cases.

(2) The 2nd cheapest for small amount is to use instant exchange such as changenow, changelly, etc., but it has a bad reputation on this forum.
(3) The 3rd cheapest is to use centralized exchange the likes of Binance et al.

For (2) & (3) you do it when the gas price is low.