When you are in Sapa and want to get to Loas you can book a ticket at different shops. We booked ours in the Hao Lan hotel. (also named Diamond Hotel / barbershop, hairsalon) They can offer you a busticket between for $22. We also visited other places and asked around but this was the most cheap bus we could get to Loas from Sapa.
When you are in Hanoi and want to get the bus to Sapa just walk into a travelshop or ask your hostel or hotel. They can offer you a busticket between $10 and $15. I took the sleepingbus at night. This nightbus will pick you up in Hanoi at 21.00 in the evening and you’ll arrive in Sapa at 6.30. (9.5 hour busride) The bus is nice and you have your own bed. Big luggage will be in the luggage space in the bus.