Once among the sports of the affluent, tennis still evokes images of manicured grass courts, sparkling white togs, and exclusive racquet clubs. But the era of starchy tennis "whites" is long gone. These days tennis is being embraced by the common man. And when the common man has laid out a hefty sum for a sophisticated titanium racquet that's supposed to improve his game exponentially, he wants a lot of choice and a great deal. At the Sans Souci Tennis Center in North Miami, that's what he'll get. A dozen well-maintained hard courts (and one clay court) provide a pick of surfaces available from 8:00 a.m. to 10:00 p.m. on weekdays and from 8:00 a.m. to 8:00 p.m. on weekends. Those who can't tell the difference between forehand and backhand can take group or individual lessons from several pros. And loners who want to work on their serve have the option of whacking balls on a two-sided practice wall. All this comes for the pittance of $2.50 per hour during the day and $4.50 per hour at night, or $1 and $2 respectively for North Miami residents. First-class facilities for ordinary folks.