The post assumes that the readers already have some basic knowledge of Lao and Thai.
As of August of 2015, 1 US dollar equals to 8175.95 Lao kip. When shopping in Laos, you will have to know how to say ten thousand.
10,000 - (หนึ่ง)หมื่น vs สิบพัน
10,000 - (ໜຶ່ງ)ໝື່ນ vs ສິບພັນ
10,000 - (nueng) muen vs sip phan
20,000 - สองหมื่น vs ซาวพัน (Lao ซาว sao = Thai ยี่สิบ yi sip)
20,000 - ສອງໝື່ນ vs ຊາວພັນ
20,000 - song muen vs sao phan
30,000 - สามหมื่น vs สามสิบพัน
30,000 - ສາມໝື່ນ vs ສາມສິບພັນ
30,000 - sam muen vs sam sip phan
90,000 - เก้าหมื่น vs เก้าสิบพัน
90,000 - ເກົ້າໝື່ນ vs ເກົ້າສິບພັນ
90,000 - kao muen vs kao sip phan
The word muen which means "ten thousand" exists in both Thai and Lao. In both languages numbers ranging from 10,000 to 99,999 are built on this word, as illustrated above. However, from my experience traveling to Vientiane, Laos, it seems that it is more common to use, literally, "ten-thousand" which is sip phan than muen. But if you use muen, people will still understand you. I did not go to other Laotian cities and provinces, so I do not know if this generalization can be applied to the whole country.
In contrast to (Vientiane,) Laos, in Thailand it is more common to use muen, and many people will be confused if you use sip phan. So, while in Thailand you might not want to use sip phan; do not expect everybody to understand you if you use sip phan.
Since it is more common to use sip phan in Vienetiane, Laos, whenever you go to Vientiane, you might want to consider using sip phan and not muen, even if you will still be understood with the latter. Using sip phan might make communication smoother. I personally like to try my best to talk like a local wherever I go in order to fit in (and I sometimes try my best to sound as differently as I can whenever I do not want to fit in). It might also be easier for you especially if you are a native speaker of English. In English, we do not have a word that means "ten thousand" like muen; we instead have to use two words to say it. So, you can just translate the numbers in this range word-for-word from English.
As for 100,000 I am not completely sure on this, but I believe that the people of Vientiane use saen (แสน/ແສນ) just like Thai people. So, for 100,000 and higher, you might not be able to translate word-for-word from English. If you have information on this, please do let me know! :)