He should've said either "one 10 hour shift per month will make 30% of what you need to live here" or even "one 10 hour shift per week will make more than what you need to live here."
I think hé means one should do all kinds of small projects.
$17 x 26 h = €442
One 10 h shift per week is to much apparently.
> In this case, you could work just one ten-hour shift per week, and in so doing, earn more than 30% of what you need to live well at this particular house with just four days of work per month.
What he probably did was write that one shift is more than 30% of what you need, then switched gears to write about four days of work per month, but forgot to remove the 30% number.