That's virtually impossible

That's virtually impossible.

That's virtually impossible to tell, because it varies wildly, depending on a lot of different factors. You could start off by doing some simple math, assuming a revenue of $2 per 1,000 views. What you need to know (and what you didn't include in your question) is the total number of views per month. Because your assumption of 300k visits doesn't tell anything about how long it takes to acquire those views. Even old videos continue to be watched and to earn revenue. But let's keep it simple and assume the guy gets 600k views per week (which is 2,4M views per month). That would mean a total revenue of slightly less than $5,000 a month. But there are more factors in play.

First of all, we must consider where they money actually comes from. YouTubers are paid from advertisements, which are sold on an auction base. The highest bidder gets the best spot for his ads. But of course, there are times of the year when advertisers are more interested in advertising. During the summer months, there aren't a lot of advertisers. During the holiday season, advertisers are falling over each other to get their ads out there. So all that highly influences the CPC (Cost Per Click) that a YouTuber gets. Basically, in december I get at least 3 times as much for the same amount of views than I get in July.

Secondly, most big YouTubers are partnered with networks, which take a share of the revenue. For big YouTubers, this may be 10% or 20% (for smaller YouTubers it's usually a bigger slice of the cake).

Finally, for a lot of YouTubers the advertisements are their smallest source of income. They also make money from sponsorships ("this video is brought to you by..."), selling merchandise, product placement (companies pay them to show products in their videos) and affiliate marketing (referred links to webshops). If you have a huge reach, that means you are very interesting to advertisers, who gladly pay you a little extra to get featured on your show.

So in the case you've presented, I think this YouTuber would earn at least $4,000 a month. But his total income from YouTube (including sponsor deals and external advertising) might just as well be a 5 figure number.

Well, that’s a very open-ended question, but I would try my best to answer it. 
Firstly, let’s debunk some of the popular myths about how people earn from YouTube.
Myth #1: More subscribers= More revenue
Reality: There’s absolutely no relation between subscribers and revenue. You can have 100K subscribers, and still make 1000$, and someone could have 10,000 subscribers and still make 1000$. 
Why?
I’ll explain it soon.
Myth #2: More views= More revenue
Reality: The number of views might be a good enough metric to decide how much a video is going to make, but it’s still not a solid metric to base your calculations.
Doesn’t make sense?
Well, allow me to explain.
 
YouTube Revenue Model Explained: 
 
YouTube does not pay on how many subscribers you have or how many people share your video. Instead, it pays you from the share it earns from showing ads on your video. 
 
The metric that you should actually consider is CPC. CPC (Cost Per Click): The amount Google makes every time an ad is displayed. Now, the tricky part is that CPC is not fixed. It keeps varying, and depends on many extrinsic factors like timing, content and affiliation.
 
For example, as Geoffrey Reemer mentioned in his answer, the CPC of ads increases during holiday times, and drops during a certain time of the year. 

Google has fabricated its YouTube revenue model as something like this:
 

• It takes 45% of revenue received from Google ads
• The rest 55% is given to the YouTube channel, which is then split between the YouTube partner and video creator.

So, the amount of $$$ that you’ll make depends not on the number of views or subscribers, but based on how astutely your partner places the ads, and how well they are received. 
 
According to fair estimates from a popular advertising blog, here’s how the stats roll out based on the type of ads:
 

• Pre-roll ads: $5/1000 views
• Banner ads: 0.80/1000 view

Assumptions: 
 

1. You have affiliated with a YouTube partner. As      a matter of fact, you do not need to partner with a network to start      earning. Read more about how you can apply,here.
2. Your YouTube channel has 1 million      subscribers, and your channel a conversion ratio of at least 40%, which      means at least 400K subscribers would watch your video.
3. Banner ads and pre-roll ads are normalized.

Total earnings= (Conversion Ratio * Number of Subscribers) * Ad-specific CPC
                        = ((40* 10,00,000)/100) * ((0.80+5)/2)/1000
                        = (40* 10,000) * 2.9/1000
                        = $1160
 
That’s just a rough estimate though, based on my above assumptions. You can earn a lot more by partnering with the right network and normalizing ads for maximum conversions.

It often doesn't really matter how many subscribers a YouTuber has, but more importantly how many views they have. That, combined with CPM (Cost per 1000 Impressions), which is how much advertisers pay to reach 1000 viewers, is a good estimate of how much money they make. 

The equation that calculates a Youtuber's earnings is: 

(Credits to ReelSeo)

So, in your example, if someone is making 2 videos a week with 300K views each, they get about 2.4 Million views per month. And if we take the average CPM, which is around $7.60, and put it all into the equation it turns out to be: 

                $7.60 x (2.4 Million views/1000)   =    $18,240/month

So a Youtube with 2.4 Million views per month makes about $218,880 annually!