Ive managed to find out half this question, so here it is.
A bus leaves Sapa at around 8am from the main intersection just outside Sapa to Dien Bien Phu ( 100,000d)
Buses leave Dien Bien Phu around 5.30am on Mon, Wens and Fri for the border (Tay Trang) cost 75,000d.
You then need to cross the river 2000kip and then walk to the bus station which will take you onto Muang Khua.
From here you can get a bus to Udomsai not sure of times or cost.
#2 thegingerbird has been a member since 30/4/2008. Posts: 23
Like most places in Laos they claim an 8 am departure but I remember it as being slightly later. Arrive early, reserve good seat with personal belonging left in seat. Two or three bucks, not four I'm sure. If your not rushed Muang Kua has a sleepy charm that many find worth a day or five. Not so many older towns that haven't been over run. Steep stone paths leading up between houses to the market. Good blog entries here http://laomeow.blogspot.com/search/label/muang%20khua
from Ms. Lao Miao. She devotes 15 entries to the place.