Just came back from Chiang Khong, and had a similar experience but with a different result. Officers asked for ฿1800, I said I only have $40 and persisted. They said if you don't enter the country you must pay ฿1800. I said no problem, I will enter the country, turn around, and exit the country. They acted surprised at the idea, but eventually gave up. They had me fill out the forms, I walked over to the bank window, paid my $40, walked through the gate, turned around, and walked to the departure booth and on to the bus.
So it seems ฿400 (฿1800 - $40) buys you:
- No forms to fill out
- Shorter turnaround time with no arguments
- Saves a ~50m walk through arrival and departure
- Saves a page in your passport since they don't stick a visa in it
If that's worth ฿400 to you, then don't bother going the $40 route. Otherwise, know that $40 is still possible if you insist.