There are night special DTC buses , which have specific timings from the Airport to New Delhi Station.
The fare is less , viz 64 INR- 80 INR (??).
But check the timings from the Official website of DTC.(www.dtc.nic.in).
This might be the cheap, but not the best option.
Autos...
more... are available, but there's a probability that you might nt get it from T-3.
They charge 350-450 depending upon your negotiation skills.
Prepaid taxis which you will find at the exit are better option, albeit not the cheap as they cost around 500 INR, but probably the safe and best option.