Carnegie Deli 5th and 7th
Sarabeth's Restraurant: 59th and Central Park South--this is good food in a nearby location
Palm Court at the Plaza - 59th and CPS --offers breakfast lunch and afternoon tea-a la carte brunch on Sunday - spectacular open room in the Plaza Atrium --tea on Sunday would be 31- 50...honestly this is well worth it as I think your mom would love this if she likes hotels and old New York
Oak Room and Oak Room Bar at the Plaza Hotel ) entrees around $32--Oak Room Bar looks out on Central Park
The Algonquin Hotel brunch (one of my fave old hotels--where Dorothy Parker's Roundtable was held). This is also pricey for Brunch --25-35 or so
Brunch at Carlyle Hotel - 76th and Madison -$59 prix fixe brunch