My suggestion will be to reach Rishikesh or Haridwar in Uttarakhand from any part of India and then take a Uttarakhand Government or a private Bus to Sonprayag.
Please Note that the direct bus service to Sonprayag from Rishikesh / Haridwar are very limited & Prior booking is recommended.
I will share the route I took to save time & cost.
From Rishikesh to Rudraprayag & then to Guptakashi, I travelled on a local private bus and from Guptakashi to Sonprayag I travelled on a sharing cab.
You will find many budget friendly restaurants on the way, so if you feel you need to take a break, you can take a halt in Rudraprayag or Guptakashi or Sonprayag.
Please note, if you have booked a Helicopter service in advance and your helipad is in Phata, then you have to get down at Phata which is on the way before Sonprayag.