So for your example, a 2 unit with a $1400 rent roll. Right off the bat, I would need to see what the unit mix is. Typically in Allentown you'll have a converted row home that has a 1 bedroom unit on the first floor and a 3/4 bedroom unit on floors 2 and 3.
Rent is going to be largely dependent on not only the unit mix but also the quality of the finish product.
For a nice finish product on a typical 1 bed/1 bath unit in center city you can expect up to around $800 ($850-875 if the unit is extra nice or has something else going for it, like being super large, exposed brick walls, etc.) The 3 or 4 bedroom unit can rent for $1100-1200 in similar condition assuming new updated kitchens and baths.
So what might be advertised as a $1400 rent roll, may very well be able to be $2000 with less than $10-15k in work. A savvy investor may look at the property and realize a $10,000 investment in some new kitchens and bathroom updates may net them $7000 a year in additional rent. This may be happening with some of these properties. I know you're taught to generally buy based on in place rents, but real solid market knowledge can make you comfortable with deviating from that rule in certain situations.
I would underwrite your deals with 1 month vacancy, $700-1000 per unit in repairs and capex, 10% management, and accurate taxes, $375 per unit for trash annually, $75 per unit rental fee annually, $20 per month for common area electric, $800 per unit if you pay gas heat and hot water, $1400 per unit if you pay oil heat and hot water for tenants. $850 for insurance for duplex.
Also gotta keep in mind that a lot of people buying duplexes are self managing and not including management fee in there, which is going to put you at a disadvantage.
