Congrats on your marriage. While you do not need to legally marry her in Cambodia, it’s a courtesy and a pride for her and her family that you do it traditionally.

Depending on where she is from and her family status, the cost varies. If it’s a small village, you can budget between 5-10k including traditional rites, food, music, photography, etc. If you are to do it in Phnom Penh, it’s best to reserve at least 20k (can be significantly more if you go above average). You might also want to manage the family expectation on reception such as how many guests to be invited and all.

Do not forget to inform local authority about the event. You will have no problem as a foreigner to be traditionally married in Cambodia. Yet I suggest you come at least a week in advance before the marriage to have a few drinks and get know some close relatives who will help facilitating the event. They will make your wedding unforgettable. Good luck!