*moderate* priced lunch near met museum?
One option - if you are going to the museum - is the cafeteria itself, in the basement, which has quite good food.
Several coffee shops on Madison - Nectar (79th & Madison), as well as Viand and Three Guys, a little further down. There's also a Three Guys and Amityville around 88th & Madison.
There's a Pain Quotidien on Madison and maybe 84th. If you just want a quick bite, you could also pick up something at Dean & Deluca and sit at the bar in the window of the store - 85th & Madison.
Ottomanelli at Lex & 93rd has a great hamburger and waffle fries and is v. reasonable. If you are willing to head up a little further even, you could stop into El Paso Taqueria on 97th, just west of Park.