I don’t know which is more remarkable: the price tag or the appreciation. The 18,500-square-foot, 103-year-old Henry T. Sloane Mansion at 18 East 68th Street just went on sale for $64 million, the most expensive officially listed house ever in New York. I thought that rocker, Lenny Kravitz, had set the unbeatable bar a couple of years ago when he paid a reported $40 million for the Duke-Semans mansion on Fifth Avenue. But since then there have been several townhouse sales in the $50 mil range. Not surprisingly, many of them are owned by weasels financiers, probably paid for by