Santana
If your papers / bikes are in order,there's no need to use an agent in Chiang Khong. You pay 400 baht on the Laos side, "nothing" on the Thai side.
BUT you say one of your bikes does not have any registration, so you've got no chance of getting the bike out unless you use an agent & pay money.
5,000 baht has been the going rate in Chiang Khong for agent fees for many years.
If you look at it as 5,000 baht for 2 weeks in Laos, it is not that bad, especially if your bike is not legal & you should not be able to take it out of the country.
Now if you do get the illegal bike out of the country at Chiang Khong, then unless you return to Thailand via Chiang Khong, you will have a problem trying to enter Thailand at another port with the bike as it is illegal cannot go in the computer.
You might want to take a look at
https://www.gt-rider.com/crossingborders.html
for more info.
Also if you do a search for "entering Laos" you will turn up
http://board.gt-rider.com/topic.asp?TOP ... rms=bridge
The topic is called Entering Laos at Huay Sai & should be a big help.