Subway Link: Both Builders And Riders Get a Bonus
Transit Authority is about to restore link between two subway stations on Manhattan's East Side that was severed in May, and with it, ability to transfer between two; transit officials say that two new escalators linking 51st Street stop on Lexington Avenue No 6 line to Third Avenue stop on E and F lines will be ready to use on Sept 6, 20 days ahead of schedule; early opening is result of authority's first, small foray into incentive contracting; Gottlieb Group bid $10.5 million and pledged to finish escalators in 125 days; though company earned $700,000 in bonuses, net cost to authority was far less because it will save about $520,000 in costs stemming from keeping escalators closed; photo (M)