Indiahhh
First: you must see the Taj. I've been there five times and honestly it is amazing. Since they've built a better road to Agra it's easier and faster than ever to get there.
Second: where you fly in and out of does depend on where you will be and what you want to see. Lots of flights from the West into Bangalore and Chennai, and these tend to be cheapest, after Delhi. What I might suggest is to fly RT to Delhi, then take Indigo (return) to Bangalore. The difference in fares into the country might almost pay for the Bangalore flight. Then move around the South (take the Shatabdi train to Chennai if you want to go there -- a great experience) and return to Delhi from Bangalore. It's easy to get to Chennai or Trivandrum or most anyehere else in the south from Bangalore. IMHO