Hi, if you want to reach Spiti from Dehradun by public transportation you can follow this.. 1. Take a local HRTC/ Volvo bus from Dehradun to Manali. It'll take around 8-10 hours and cost 1000 for Volvo. 2. Take a local HRTC bus from Manali to Kaza in the morning. You'll reach Kaza in the evening and it cost around 400 rupees. Hope it helps you and do share your experience on Tripoto.
Dehradun to Manali and then the morning bus to kaza is the shortest and cheapest.. from kaza you can take the bus to Manali and get down at gramphoo.. which is before rotang and take the next leh bus..