Penthouse 9 at 50 Battery Street on the Boston waterfront last traded way back in April 1996 for $715,000 (or about $1.1M in today's cabbage). The 2,421-square-foot, 4-BR, 3-BA with a wraparound terrace is back on the market as of Tuesday through Otis & Ahearn. Its tag this go-round? $6,300,000, or a stonking $2,602 per square foot.
The price includes two garage parking spaces, and the duplex's location is at the nose of the complex, so it's pretty much water views for as far as the eye can behold.