I just crossed from Thailand to Cambodia today at Koh Kong.
I'd done my research and expected the visa-on-arrival to cost $30 USD. WikiTravel and WikiVoyage vary a little. One says $30 or $35 for a visa on arrival. The other says $30 for a visa-on-arrival vs $37 for an eVisa, specifically. This is one of the reasons I didn't bother getting an eVisa.
When I got to the visa a man in a kind of uniform but possibly some kind of semi-official tout told me I couldn't pay in USD and that it cost 1,500 THB (about $50 USD). I expect scams at Cambodian border crossings and didn't believe him and told him so. He took me into the actual immigration office where there were two more men in genuine uniforms. There was no list of fees or anything I could see on the walls or desk mentioning any prices at all. I didn't have to pay the ridiculous 1,500 THB but I did have to pay $37 USD, 7 dollars more than I expected or is documented. I don't believe I got a receipt. I will dig through my documents and add any extra info I find.
This doesn't mean I was scammed, necessarily. It may be a legit fee increase. They may be playing on the confusion and difference between the visa-on-arrival price and the e-visa price making it a "plausible" scam. But the 1,500 THB first offer makes me doubt it was legit.
Can anybody confirm what the official rate should be from an official source? I'll also try to find info on forums about people discussing fee increases or border scams.