It depends on really many things.
Are you Russian? If so you can buy much cheaper products directly at Steam, key resellers like G2A or Yuplay (yuplay usually provides region free keys, anything else is locked only to Russia)
If not above, then head over to gg.deals (it is full site address) - it will show all available deals on many platforms (Steam, GOG). If you're console player, then you may want to search on eBay for used games - do not look for digital ones as they are usually the same price as new copies and also you won't be able to resell the game.
You might also try to get on lowcygier.pl - it's polish site, but pretty straightforward - title, discount and direct link.
There might be also people selling account access to new games for less than 1$- it's not fake, it's real, but well ... it's not cool