If you sell your game through Steam, you cannot have it cheaper anywhere else. Even DRM free version on your website. Even temporary sales.

I don't see anything about this in their docs. The closest is "You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important that you don’t give Steam customers a worse deal than Steam Key purchasers."

That seems reasonable. If you're selling DRM-free, you don't generate steam keys, and valve has no stake in it.