Most of those buildings next to what used to be the highway are at most 10-20 stories. A 60 story skyscraper was finished in Boston this year for $700[1].
So to a first approximation they could have had a full 2-3 blocks of parks around 15 new such skyscrapers to get the same building space after demolishing most of the buildings seen in the foreground on that picture, and have come out even on floor area.
They'd then have had had $12 billion left over to elevate the highway and put that elevated highway in a tunnel to cancel out the noise before they got to to $22 billion.
1. https://en.wikipedia.org/wiki/Four_Seasons_Hotel_%26_Private...