I have been staying at SAYSAMONE Guesthouse. It is directly on the Mekong River. AC and single or two beds for around 10 usd's. Very nice and great relaxation.
I went on a day trip, southbound on the Mekong. For eight hours we (two persons) spent 25usd total. We did swim in the waterfall, and had lunch in a village. I can tell you that the return trip (4 to 6) was worth my 12.50 and more. The river comes alive after 4pm. The tide has risen, children use steep mud banks as a water slide, woman wash themselves, and the men pull in the fish nets. Wow! It was awesome. The boat I took was skippered by Mr. Am Kha. He is located down near the Penninsula Massage where the Mekong meets up with that other river. Sorry I don't recall the name.