Who rents those huge Soho lofts with asking prices to make a financial planner do a spit-take on a tall glass of Alka-Seltzer? Celebrities, of course, who parachute into town for a few months at a time and desire a home in the heart of the action. The latest, the Post's Jennifer Gould Keil reports, is pop star Rihanna, who'll shack up for the summer inside a 2,300-square-foot 3BR spread in the newish Mercer Greene at 92 Greene Street. According to Keil, RiRi will pay $18,000-per-month for the apartment, which is listed for $22K/month. Sweet, a bargain!