There are regular buses from Luang Prabang, Vientiane and Vang Vieng to Phonsavan. It takes between 8 to10 hours, depending on the city of departure, and the prices of the buses are around 20 USD per person.
From Phonsavan, you can rent a day trip through your hotel or guesthouse to explore the attractions, which will cost about 25 to 35 USD per person, which will cover your transport as well as a guided tour.
You can also rent a taxi or hire a motorbike to explore the sites on your own. The price varies between 30 to 50 USD for a car per person, and a bike on rent costs about 15 USD per day.