Maybe 25% of the gross rental receipts monthly?
I would be willing to pay someone that much to take care of our place while we’re away.
You know what the usual income is, what expenses usually run and how much the owner paid.
Figure capital cost at about 5% of the sale price of the duplex, so that would be about $100/week per hundred K.
Add this to the other expenses, subtract from projected income, and you have the profit margin, assuming that nothing changes.
You’re doing all the work so you deserve a significant share of the profit.