Drivers of jumbos and tuk-tuks will take passengers on journeys as short as 500m or as far as 20km. Understanding the various types of tuk-tuk is important if you don't want to be overcharged (and can save you arguments in addition to money). Tourist tuk-tuks are the most expensive, while share jumbos that run regular routes around town (eg Th Luang Prabang to Th Setthathirath or Th Lan Xang to That Luang) are much cheaper, usually less than 5000K per person.
Tourist Tuk-tuks You'll find these loitering in queues outside popular tourist spots, such as at Nam Phu. In theory, chartering a tuk-tuk should be no more than 20,000K for distances of 1km or so, but these guys will usually show you a laminated card with a list of fares that are at least double what a Lao person would pay. Bargaining is essentially fruitless because there is an agreement within the queue that tuk-tuks won't budge from the agreed tariff.
Wandering Tuk-tuks These tuk-tuks will pick you up anywhere and negotiate a fare to anywhere. Prices are lower than tourist tuk-tuks and rise as you head further away from main roads. If you're going somewhere within the centre of town, you can probably get away with handing the driver 15,000K to 20,000K and telling him where you want to go.
Fixed-route Share Jumbos The cheapest tuk-tuks are more like buses, starting at tuk-tuk stations and operating along set routes for fixed fares. The biggest station is near Talat Sao and one very useful route runs to the Friendship Bridge (5000K, compared with about 200B for a charter). Just turn up and tell them where you want to go.