what is the taxable value under GST.?

I am an air travel ticket booking agent( middleman between air travel agency and client). I'm purchasing tickets from travel agent and selling it to clients by adding my margin. what will be my taxable value.?
Ex:- Purchasing ticket from travel agent @ Rs.10,000 and selling it to client @ Rs.10,500. That 500 rupees is my margin and i'm paying gst for this as this amount includes gst.(taxable value - Rs.423.73(500×100/118), cgst and sgst- Rs.38.13 each). so should i take Rs.423.73 or Rs.10,423.73 as my taxable value.? please help.! Thanks in advance.