This varies greatly between YouTubers and depends on the audience and type of content.
PewDiePie's audience is mostly 12 year old children without a disposable income, while it's not unheard of, he very rarely generates enough interest in a game to cause a spike in sales (like he did with Skate 3).
His viewers don't go to his channel to get information on games to make a better purchasing decision, they go there to be entertained by his antics.
TotalBiscuit on the other hand has an audience that is mostly 25+ so likely has a job and therefore disposable income.
His videos are made as a buyers guides, people go to his channel to get information on games.
This is what is important,
audience. PDP has 43million+ subscribers but his impact on sales is negligible, TB has 2.2million,
twenty times less, but there are several games that had 0 marketing budget and managed to sell on his video alone.
One example of the top of my head is AntiChamber. The only noteworthy coverage the game had at release was TB's video (and I think there was an article on RockPaperShotgun). The developer said in a
Polygon interview the sales in the first 24 hours blew peoples expectations.