Duplex pencils at 380 in month one and 190 after I leave. Which number am I supposed to buy on?

You buy on the number you'll be living with longest, and unless you're planning to occupy that unit for a decade, that's the 190 negative. But your post-move-out figure is built on a fixed payment against rents you've assumed static, which is the part I'd push on before deciding.

Run it forward. At 3 percent rent growth, 3,550 gross becomes about 3,880 in three years and roughly 4,110 in five. Your PITI moves with taxes and insurance, not with principal and interest, so call it 3,150 growing to maybe 3,350 on tax and insurance drift. That flips negative 190 into modestly positive somewhere around year three or four on those assumptions. The whole case rests on rent growth continuing in your specific submarket, and if it stalls you own a property that eats 2,300 a year plus a roof.

The roof is the item you've handled worst. Four to seven years on a duplex is not a reserve line, it's a scheduled capital event, probably 15k to 25k depending on square footage and material. Add 300 a month against it and you're negative 490, which changes the question. Either price that into your offer now or get a contribution at closing, because you won't have refinance equity to draw on at 5 percent down.

The other thing your model skips: 8 percent maintenance on a building where you're the occupant is not the same 8 percent once you're a remote landlord fielding calls on both sides. Turnover costs on the unit you vacate tend to run higher than owners expect because you've been living in it, not maintaining it as a rental.

If the seller won't move on price, the honest read is that you're paying for one cheap year and buying a mediocre rental to get it.