Affordable does not mean inexpensive.
It's a common mistake, so I'll be glad to define that for you:
Affordable housing in the US is usually defined as the total rent or mortgage payment being less than or equal to 30% of household income.
The usual classifications are moderate income affordable, low income affordable, and very low income affordable.
Determining where your household falls in those brackets is based on the average median income (AMI) for that MSA (metropolitan statistical area).
The 2015 Area Median Income for a family of four in Marin County is $101,900 (http://affordablehousingonline.com/housing-search/California/Marin-County/)
moderate income households earn between 80% - 120% of AMI
low income is 50% - 80%
very low income is less than 50%
So to be affordable for:
moderate income household the units would need to be priced at between $2038 - $3057 per month
low income household the units would need to be priced at between $1237 - $2038 per month
very low income household the units would need to be priced at below $1237 per month
Usually we're looking at rental prices rather than mortgage payments, but it can be performed either way.