International passengers doing domestic transit can opt for COVID-19 test, waiting lounge at Delhi airport for Rs 5,000 @ http://www.financialexpress.com/life...-5000/2081657/

Appointment booking (and a FAQ) is at http://www.newdelhiairport.in/covid1...ntment-booking

The FAQ says it takes 4 to 6 hours, during which presumably you need to wait at the Plaza Premium lounge. So your connection time would need to be at least 8 hours? The Plaza Premium lounges are all priority pass lounges (but IIRC, they restrict the length of time you can stay), so I'm assuming the cost of the test would be Rs 2,400 (no Rs 2,600 lounge fee) if you have a PP.