I did a project in grad school where I costed out the Main Street theatre, and both a hard-ticket premium show for the evening, and a Mickey and the Wondrous Book-style show to relieve capacity during the day. The former with a 30-piece orchestra. More than most Broadway shows, but less than Candlelight has (50 + the conductor, so 51). I look for quite a ways to figure out how much Disney paid their orchestra members, as they are unionized. Closest I could find is during the Pixar orchestral show from several years ago at DHS - each member was being paid
$285 a night. At that rate, a 51 piece orchestra costs $14,535 per night, or $58,140 per week. The average dining package for Candlelight cost $75.50 per adult last year. Assuming we're being conservative, and say that Disney fills up 30% of the theatre with paying customers... That's $22,650 per show - 3x a night makes this $67,950. IE for every night they do it extra, they pay for a week's worth of orchestras, plus a hefty profit. Even if you assume a hefty cheque is passed on to the narrator, and even if the wage paid per night to the orchestra has substantially increased per night (to $300/person or more), Disney pays it off in 2 nights a week maximum. Suffice to say, it's nowhere near as expensive as some might think, especially with the dining package. And that was being conservative- fill it up with 40-50% or more, and the money just keeps on coming... More than enough to pay it all off.