Cat scales are very accurate. They do not weigh individual weights as to 1# increments. It is usually in 5# increments. They also stand behind their scale tickets. If you , as a trucker, get an overweight ticket, they will stand behind their weights and when you go to court will back you up.
The diagram earlier posted is a very accurate description of the way a cat scale operates. It is very easy to weigh a short trailer. Just make sure your front axle is over the break in the pads, and the rest will usually fall in line to weigh properly.
The cost varies from location to location. It is somewhere around 10-12 dollars for first weigh and then a small amount for the reweigh.