This is one of the cheapest places in Southeast Asia to sit still, and the two things that actually cost money are getting here from Pakse and the two waterfall tickets.
Getting here is the expense
The public bus from Pakse to Nakasang is around 40,000 kip and takes about three and a half hours; a minivan does it in two and a half for more. Add the longtail across to Don Det. From Vientiane it is an overnight sleeper to Pakse arriving around 6am, then the same onward leg — there is no railway south.
The waterfalls are the only fixed tickets
Khone Phapheng, on the mainland, is around 55,000 kip plus whatever the tuk-tuk costs to get there. Li Phi on Don Khon has its own smaller ticket, collected at the bridge along with the island access fee. Everything else you might do here — walking, swimming, watching the river — is free.
Bring kip, in cash
Do not count on an ATM or a card machine on Don Det or Don Khon. Draw cash in Pakse before you come south, keep small notes for boats and bicycles, and assume your guesthouse bill will be settled in kip at the end of your stay.
A fan room is a third the price of a view
The cheapest rooms are set back from the water with a fan and a shared bathroom; the same guesthouse's river-facing bungalow with a hammock and a private bathroom costs several times as much. Since you will spend the day in the hammock, this is the one upgrade genuinely worth arguing about.
What a day costs
Reckon on a basic room, three guesthouse meals, a bicycle for the day and a couple of large Beerlao. The two numbers you can plan around are the 40,000 kip bus from Pakse and the roughly 55,000 kip Khone Phapheng ticket; on a day when you do neither, this is about as cheap as travelling gets. Electricity and wifi are patchy and no cheaper for it.