In Chiang Mai, we were told just to just get USD for Laos as that's the common currency. This turns not to be not true. Some bigger places do accept USD or Thai baht at a worse rate., but most places do expect you to pay in Lao kip.

Lao kip bills have both Arabic numerals and Lao numbers on it so it's a bit confusing to see what the value is. Make sure you familiarize yourself with the bills. A local tour guide told us the value is always the smaller number. Keep an eye on the change you get from vendors.
There are lots of ATM's around town. I believe the withdrawal limit is 1 million kip per transaction if you have a foreign card. With my Union Pay card, I was only allowed to withdraw 1 million kip and had to pay a 15,000 kip transaction fee. Not all machines take Union Pay so you need to only use ones with the logo or it will just spit out your card.

Based on prices we saw in Luang Prabang, this is what you might spend per day, as of February, 2014 :

meals x 3* ₭ 15,000* - 50,000*+
Main dishes start at 15,000 kip, even at local-looking restaurants. Savory or sweet crêpes are 10,000-25,000 kip. Western dishes or mains at slightly nicer restaurants start at 30,000 kip. You can have a gourmet meal for 100,000 kip. If you want very cheap food (2000-5000+ kip), you can buy small packs of cooked food at the market where locals actually shop.

Although we quite often eat for around 50,000 kip for 2, including drinks, a meal for 2 with drinks at a decent restaurant is around 70,000 kip. An extravagantly nice meal for 2 at a riverside restaurant is under 300,000 kip, including drinks.
snacks & drinks ₭ 5,000 - 25,000+
5000 kip for 1.5 L bottled water. Canned drinks are usually 5000 kip also. Fruit shakes are around 6000-10,000 kip, same for a large beer. Although some restaurants charge a bit more, but strangely, prices are the same on the streets as in restaurants.

Locally made snacks can be bought at little shops for 1000-2000 kip.
Transportation ₭ 0 - 50,000+
It's fairly easy to get around town on foot. You can rent bicycles for 15,000-20,000 kip a day — you need to leave your passport behind. Mountain bikes are 50,000 kip a day. Motorbikes can be rented for 120,000-160,000 kip a day.

Private tuk tuk to nearby attractions range from 170,000-250,000 kip — a shared tuk tuk will be cheaper, but sometimes you'll need to wait til the tuk tuk fills up with passengers.
Accommodation ₭ 40,000 - 300,000+
A dorm bed with a fan is 40,000 kip. A room in a guesthouse with ensuite bathroom and fan is from 80,000 kip, or more than 100,000 kip if you want a balcony. A nicer simple room with ensuite bathroom and aircon start at 150,000 kip. You can find pretty nice riverfront guesthouses for $35-50+ USD.

If you stay more than a week, prices could be negotiated — 1 night free or perhaps a discount.
Other Expenses ₭ 8,000 - 168,000+
Laundry is 8000-10,000 kip per kg. Internet cafes are 6000 kip an hour.

Massages start at 40,000 kip per hour, although the "Mystic Lao Massage" I had for 60,000 kip at a beautiful spa called The Spa Garden was very average.

You can only get o.b. tampons (without applicators) here : 25,000 kip for 8 regular ones & 30,000 for super.

If you want to join a tour, expect to pay 20-50+ USD for a day trip. Also, don't forget the various admission fees, usually 20,000 kip each.
TOTAL ₭ 98,000 - 693,000+ About USD $ 13 - 87+

In Luang Prabang and Vientiane, we are living quite comfortably for roughly 250,000 kip (~30 USD) per person per day, all inclusive.

In Vang Vieng, we spend under 200,000 kip a day per person (~25 USD) including everything.