Lao Tourism Office in Phongsali can set things up in Phongsali. It is a gov't-sponsored service and I have found most of these offices, including in Phongsali, to be quite competent if you are a little patient and have a realistic understanding of costs.
Quality trekking is not particularly cheap. Personally, I want to minimize the number in the trekking group (2-4 is ideal, I'd say). Second, I want a quality translator guide - one who knows the village language and has knowledge about the region and the peoples. If they can facilitate good meals with good cheer with all involved that's worth some too. A good guide knows the villagers to assure all involved parties are well-treated and respected, and that a positive experience is mutual for all.
I like curious, engaging, quieter guides with lots of experience, and they tend to cost more because they are certainly worth more. The prices quoted in #2 seem reasonable. More people in a group reduces costs considerably; more people also reduces the quality of the experience considerably, too.
Also, if you skimp on a guide, you're skimping on the experience. I like
That said, I have always had the luxury of arriving at a location (e.g. Phongsali, Muang Long, Muang Khua, Attapeu, Kong Lo Caves, etc.) and having a day or two to set up a trek. I am hesitant to set up a trek, say, in LP for an experience in Luang Nam Tha - meeting the guide and being "in the scene" gives you far more assurance of the quality of the guide and experience. (I do understand that many do not have the luxury of this extra day or two.) I also prefer to pay my guide/translator directly when possible.