I've purchased the Android version of Minecraft: Bedrock Edition for my son to play on his tablet then found all sorts of articles saying that purchasing it entitles one also to get the Windows version but can't figure out how.
According to Mojang Support, only players that have purchased the Java edition of the game before October 19th 2018 can get the Windows 10 edition for free:
Players who have purchased Minecraft: Java Edition before October 19th, 2018 can get Minecraft for Windows 10 for free by visiting their Mojang account. Log into account.mojang.com, and under the “My Games” heading you will find a button to claim your gift code. If you purchased Minecraft: Java Edition after October 19th, 2018, the button will not be there...
As far as I know, there is only one official way to get the game, and that is through the Windows/Microsoft store. Unless it goes on sale, I don't think there is anyway to get it cheaper.