Luang Prabang guest houses have a huge range, but you should be able to get something pretty decent between about US$15 and US$30 a night. In terms of booking ahead, there's heaps of accommodation, but when we arrived late at night it took a while for us to find somewhere that we were happy with to stay - maybe we were being a bit fussy. If you arrive earlier in the day there should be no problem finding somewhere to stay. about 10 years ago
I did the same as david - I just booked a guest house when I arrived. Currently most places say they are 'full' so in the high season you might need to shop around a few blocks off the main road. The nicer hotels charge as much as 50 euros - so there's accommodation for all styles here.
I would recommend to stay at a guest house rather than a hotel because you can get really helpful local advice and meet other travelers.
I've been staying at a guest house called 'Sysomphone Guest House' for the last five nights and I would highly recommend it. I am paying 80,000 kip ($9 USD) per night for a good room with a private bathroom. It's run by a very nice family and they even invited the guests to to drink and eat with the family to celebrate the birth of their grandchild!
You shouldn't have any problems finding a place to stay for a reasonable price in Laos if you look around. about 10 years ago