It is generally cheaper for locals in Laos to marry each other than a foreigner to marry a Lao girl in Laos. However a Lao style wedding in Laos is still alot cheaper than weddings in a foreign country.
The cost to have a traditional Lao wedding will vary from one person to another, obviously it depends on the financial ability of you as the groom, and whether you and your girlfriend plan to have a small inexpensive wedding or a large wedding with many guests.
You also need to take into account the cost of the dowry, paperwork expenses, travel expenses, daily living expenses and perhaps you want a new house and car if you plan to settle in Laos. It's a headache, there's many things to worry about so you need to have a budget set from the start, otherwise you will run into problems. Sometimes family members contribute some money, and most guests give money as a wedding gift to the couple.
To give you a rough estimation so that you have a good general idea, a simple home wedding in Laos between a foreigner and a local Lao girl cost about $1,500 US as a minimum, that includes catering and band entertainment. A general rule of thumb for simple weddings is that you should be prepared to pay $25 US for every guest on the invitation list. To have an expensive wedding in a restaurant, or if you plan to rent a wedding site, the total cost could be in the vicinity of $3,000 US and upwards.
The food in Laos isn't that expensive, it's the alcoholic beverages that makes up most of the cost and Lao people like drinking. Therefore you should have a substantial amount of alcohol to keep heavy drinkers pleased.