In Vientiane you get a 'better' guesthouse for $15-$25 (Riverside, Vayakorn). You can eat a full meal, at the cheap end of the scale, at the riverfront stalls for $4 per person, or away from the main tourist/expat areas for $1-3 per person. Tuktuks round the main central area are roughly $1 per km, large beers $1-$1.25 in most places.
Luang Prabang has similar costs for everything except the better guesthouses and restaurant food in the central peninsular area - the main tourist area. In this area you can get a small fan room from $8 but a 'better' one will run you anything from $20-50. Outside of the main peninsular area (5-10 minutes walk back from Phousi hill) though you can get a 'better' guesthouse for $8-15, and food costs drop away significantly. There is a good local outdoor cooked food market down the side streets opposite Phousi hill where you can eat for $1-2.
We travelled and never spent more than $20/25 for decent accommodation with own bathroom, and that was $20 in LP where we negotiated the price down from $30 (with breakfast), and $25 in Vientiane to be one block from the river with breakfast (roll and egg).
Always bargain. There are always more rooms than beds, unless you hit Vientiane when an event is on.
Does your trip coincide with the summer festival?
The local food is cheaper and more enjoyable than most of the foreigner restaurants. Find a good food (not trinkets) night market and try lots of options and enjoy.
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