Rather than only looking at the profitability/feasibility of the investment, it is important to consider the relative value of a project to best determine if an investment is worth your resources (money & time) and the specific risks associated with the deal. Said differently is it worth the time/money/risk/etc. to purse this development or are your resources/attention better spent elsewhere.
One way to determine the relative value of a development opportunity is to calculate a Yield on Cost ("YoC", also referred to as Return on Cost or "RoC") of the opportunity and use this as a comparison metric relative to other developments and/or acquisitions.
Yield on Cost can be calculated as stabilized NOI over total costs (e.g. $100K annual NOI / $1,000K total cost = 10% YOC). If you are considering multiple development opportunities, the higher the YoC the better the deal or the higher relative value (assuming all risks are constant such as market/location, permitting/approval risks, time to build, etc.). The Yield on Cost is also an indicator of profitability as the spread to the market stabilized cap rate is what generates equity profit (i.e. build to a 10% and sell at a <10% cap rate once stabilized, the spread/difference is your profit).
It becomes a bit trickier when comparing development projects to pure acquisitions (existing properties) given the two deals have very different inherent risks, however a general rule of thumb is that a development project should generate a YoC 1.5%-2.5% higher than the market cap rate (what it would cost to buy a existing duplex). This is because developments are riskier (zero initial cash flow, development risks, unproven rents, etc.). If the difference between the two rates is too small, or if market cap rates are larger than the YoC, than you're better off buying an existing property rather than going through the trouble of building a new property.
Hope this helps.
Adding to my initial post below...'
Doing quick back of the envelope math the yield on cost would be ~3.8% ($24k annual rent per unit x 60% NOI margin / $375k total cost per unit), which feels tight for almost any market in my opinion.