single entry evisa for VN is USD25. Cambodia evisa in advance is more expensive than Vietnam. Not sure what Cambodia's fee is on arrival but I bet it's not cheaper than USD 25
Sihanoukville is a shithole but it's where you need to go to get to Koh Rong, which is stunning. Kep is boring and the beach is crap, Kampot is a much better choice despite it being a riverside town and not coastal.