Hello dear!
Hope you are doing well!

Well in general the the cheapest way to get from Dwarka Sector 21 to Rohini is to Metro. This would normally cost you around 90 - 150 and it takes time duration of about 1h 49m. But keeping in mind pandemic situation do check the timing and fare beforehand.
If you are looking for quickest way get a taxi which costs 500 - 650 and takes 29 min.