One of biggest problems with high density living is the higher cost of buying a condo or house. That's because of supply and demand. The more people want to live in an area, the higher the price is going to be in that area.
What solutions do you guys have in keeping housing prices in urban neighborhoods affordable for the people that live in the lower and middle class?
Extend the urban boundaries. A larger network of walkable areas create greater living options for prices at all scales. For example, most only look at the urban core as consisting of Riverside, Springfield, Downtown and San Marco. Through better transit connections, urban planning, etc., Sugar Hill, New Springfield, Brentwood, Panama Park, Durkeeville, Brooklyn, Eastside, LaVilla, Robinson's Addition, Lackawanna and North Riverside can all become viable urban districts with more affordable housing opportunities.
Look at Chicago and Philadelphia. Both cities may seem extraordinarily expensive compared to Jax, but they are maybe only a hair more expensive than Atlanta and cheaper than Miami. Residential taxes in Chicago aren't even that high (nearly half what they are in Atlanta actually). They are bargains compared to NYC, Boston, San Fran, LA, and to an extent even Seattle and Portland, yet they are two of the most urban cities in North America.
River North area of Chicago is adopting policies to ensure that middle and lower income families and residents are able to afford to stay there without relocating. It's not my favorite topic so I didn't really read into it, but someone who cares can look into it :)