In front of the market in Ollantaytambo, there is a public bus, which goes from 6am to 7pm. It costs around 3-4 soles.

To go back, there is a bus from 5am to 4pm.
Here is a picture of the bus:

Private taxis will cost you 40 soles until Huilloc, 50 soles until Patacancha/Rumira-Sondormayo and 75 until Challwaccocha.