Have just checked a spreadsheet I did for our trip.
Transport - Road & Rail
From Poiphet/Siem Reap/Phnom Penh/HCMC/
Nha Trang/Hue/Hanoi = US$104.00/person
Accommodation:
19 Nights averaged US$35.00/double room/night
That's very reasonable. Did you mostly take trains or buses? How many hours did you spend travelling between places?
Bus in Cambodia, rail in Vietnam.
3 hours - Car - Poiphet/Siem Reap
Bus - To Phnom Penh - 0700/130
Bus - To HCMC - 0630/1400
Rail - To Nha Trang - 1300/2200
Rail - To Hue - 0530/1700
Rail - To Hanoi - 1500/0400+1
Bus times are not much different to rail from what I can remember. Some very scenic views on the train, daytime of course.
I looked before, Air Asia fly Vientiane/Bangkok via KUL for around US$135.00/one way(Air & Taxes) in October.