Have a question about Philly’s neighborhoods or the systems that shape them? In Philadelphia, the country’s second-poorest big city, the demand for deeply affordable rental housing is a constant. To better meet that need, the Philadelphia Housing Authority launched an ambitious — and costly — plan to remake and expand its vast real estate portfolio. That translates to an average unit cost of about $315,000 — more than the market value of many Philadelphia homes. It’s largely rooted in strict regulations for public housing authorities and the cost of construction in Philadelphia, one of the most expensive places to build in the country.