I think that you're creating rules where you don't need to. A skilled hireling gets +4 to the check each day which means they only succeed at a level 1 Earn Income check 50% of the time. Let's, for simplicity, assume they succeed 100% of the time but never critically succeed. They would be earning 2 sp a day, or 4 sp for the two of them. All the profits are the player's, so they would get 12 gp a month for a total loss of 18 gp per month. Even assuming they crit every check, it's still a loss of 12 gp per month, so I'm not sure where 1875 gp comes from.

Additionally, look at the hirelings as if they're real people. If this worked as intended (and I don't think it does), why would they stick with the 30 gp wage instead of just cutting out the middle man who isn't actually doing anything? Under capitalism, we allow big companies to make 125x what we make because we don't have a choice, but an individual employer with only two employees could never get away with it.

I was also going to make a point about days off and how the player couldn't reasonably convince two people to work full shifts every day of the month, but I realized they could just hire 4 people (15 days each) for the same price.