It's a combination of factors:
- He's been getting all the money without needing to split it with Jesse or Mike
- He doesn't need to pay off Mike's guys or Lydia
- The Phoenix crew is taking care of distribution
- They have the new money coming in from the Czech Republic
- They've increased the frequency of their cooks
After the first cook this season, Mike, Jesse, and Walt started with a split of $367,000 each before Mike started taking out the various cuts. With the new agreement with the Czechs, that total take per cook is presumably significantly higher, even if we assume he isn't making quite as much from Phoenix.
In the recap video on AMC's website, they show the pile of money a few times. It's hard to get an accurate count because it's not stacked perfectly. From a rough count: Each level of the pile is about 15 bundles by 12 bundles, and there are at least 40 levels in the pile. That's at least 7,200 bundles. The pile is a mix of bundles of $20s, $50s, and $100s, with 100 bills in each bundle ($2,000, $5,000, or $10,000 per bundle).
If it was only bundles of $20s, it would be more than $14 million. If it was an even distribution of $20s, $50s, and $100s, it would be over $40 million.
If Walt's average take was $1.5 million per cook, 30 cooks would put them at $45 million.