OK...prepared to accept that that I have poor judgement as I'm writing a more detailed post and know in advance that this is not necessarily a wise thing to do.Don't agree with this either. Either stick to your guns and pay 20USD or use a different border crossing. Going there, knowing full well you're going to be ripped off, doesn't really show good judgement. Oh, and minivans involved? Already a bad sign ...
First time I entered Cambodia via Koh Kong in 2006 (I came through Poipet on an earlier trip) , I did the tourist VOA thing. I was asked for the equivalent of $35 in Thai Baht. I refused to pay. I was told that I would have to sit outside and wait as there were many people to process. I sat for half an hour and waited. I approached the counter again and the price was unchanged. I stuck to my guns and protested. In response to my protests about the correct price for a visa, the female immigration officer said "you must realize we are poor people in Cambodia and you come from a rich country." This from a woman with so many gold rings on her fingers that there was scarcely any skin visible. I stuck to my guns for an hour and a half in total before capitulating when they dropped the price to $25. I told the immigration officer that I hoped my small contribution would ensure that her children had enough food for the day. On that trip, I paid the bogus health check fee, the 'I fill out your forms for you' tip, and the inflated taxi fare from the border into Koh Kong. I did not change USD into Riel at the rate of 3200 to the dollar despite being informed that "they don't accept US dollars in Koh Kong and you must change now" nor did I pay for gasoline when the taxi driver stopped half way between the border and KK saying that he had run out of Total (he hadn't) and it was my responsibility to buy more. I did not fall for the "Oh ...your guesthouse now closed...I take you to another one" gambit but I did allow myself to be taken to a main street booking agent for my bus ticket to Phnom Penh where I paid a Barang Levy ("because barangs are heavier than Khmer and bus uses more Total") on top of the already inflated fare.
The last few trips, I traveled through on my Normal (Business) Visa and there are no issues. I decline to pay the Health Check fee (I smile and flex my 60 year old muscles to demonstrate that I am in rude good health). I do not allow someone to fill out my forms for me. I speak a little Khmer. I do not stay in Koh Kong.
On the last trip, I traveled through from Trat with a young French guy doing his first visit to Cambodia, arriving at the border before dawn. He had a water damaged print out of his e-visa which was still reasonable legible. When Immigration opened, he went ahead of me and they refused to accept his print out saying that he would have to purchase a new visa. I left him after 40 minutes as I had a bus to catch, still remonstrating with the official. He was sticking to his guns.
I think Koh Kong is permitted to exist as a public service. As a first time visitor to the country, you are treated to an intensive crash course in some of the more venal and less salubrious aspects of the country. This training will probably stand you in good stead if you decide at a later time to come and live here.
I am going back through Koh Kong (it is my most convenient exit point) this Wednesday en route to Myanmar and it will be interesting to see if anything has changed. I doubt it.