This is an old post but I will try anyway. Take a cab from the airport to the bus terminal in Santaigo. Buses run about every 2-3 hours to Valpo. The cost should be about the same as one would expect in the states for that ride - it is about a 1 hour trip.
When in Valpo, you will see many cabs hanging out. If you are just visiting for the day and wish to tour, I would recommend asking a cab driver for a tour rate. I did this several years ago and had a personal tour guide for the entire day at a cost of about $40. (I did tip him generously). The down side is that you will likely need to speak reasonably good Spanish as few are bilingual.
Our driver took us to some of the coolest places including Vina del mar, Pablo Neruda's home, a great restuarant that few would ever find etc. The cab driver waited for us at every place he took us.
By the way, of all of my world travels, I have never met more humble, genuinely true-blue nice people than the Chileans. I have worked with them while being in the US, visited during difficult times for the country, toured and visited friends in Chile. No where in the world have I had such a warm feeling for the people in general. I hope you have a great trip.