Not sure about the bus fare but it will cost you between 30-40 Rs if you take Metro. Step out of NZM PF #07 exit, you will find umpteen number of share autos to Indraprastha Metro. Take a token at Indraprastha to Chandni Chowk, take the blue line towards Dwarka, get down at Rajiv Chowk. In the same building, you need to go underground to take Yellow Line(towards Jahangirpuri) and get down at Chandni Chowk. Old Delhi is at a walkable distance from Chandni Chowk metro. Even if you skip 1 metro either at Rajiv Chowk or Indraprastha, it should take you around 30 mins to reach DLI from NZM.