Beds, food and lagoon entry are close to free by Southeast Asian standards; the one thing that decides your budget here is how many organised adventures you say yes to.
What a day costs
Sight entry is trivial: Nam Xay viewpoint is 10,000 kip, Blue Lagoon 3 about 10,000 kip, a rice-field yoga class around 80,000 kip. A market dinner and a Beerlao is a fraction of a single tour. Budget for one splurge, not five.
The balloon is the whole budget
A sunrise hot-air balloon flight runs about $165 per person and will cost more than several days of everything else combined. It is the single decision that sets your spend here, so make it deliberately.
Scooter over organised tour
Renting a semi-automatic scooter for a day and riding the dirt tracks to the lagoons yourself is far cheaper than a seat on a buggy or minibus tour. Check the brakes, keep your passport rather than handing it over if you can, and ride slowly — the tracks are loose gravel.
Wet season is the cheap season
May to September has the lowest room rates in town, but the Nam Song runs high and brown, some lagoons close and trails turn greasy. You save real money and lose some of the reason you came.
Getting in without paying tourist rates
Minivans from Vientiane on Route 13 take three to four hours for a few dollars and drop you in the middle of town, which is often better value than the train once you add the shared van from the out-of-town station.