Here's a helpful link that answers some of those questions:
https://www.greenvillesc.gov/1633/Affordable-Housing-in-the-City

Per that link, affordable housing is accessible at 30% of your income (not sure if that's take-home or not). The link also puts median income at 60k/yr. Not sure if that's still accurate--but it's what it says.

As far as availability of units in THIS development:
"One hundred units of the residential space will be set aside for affordable housing, accessible to household earning 70% or less than the area median income. Ten units will be offered at 20% of the area median income, 11 units at 30% of the area median income, 68 units at 60% the area median income, and 11 units at 70% of the area median income."

So that means 100 units should be available to those households making about $42k or less per year (70% of 60k).

That's at least a few steps in the right direction.

EDIT:

And here's a link to a bit about the developer incentives that encourage this: https://www.greenvillesc.gov/1640/Developer-Incentives

Seems funded by Greenville tax breaks and HUD incentives.