Why and when you should use weight distribution hitch?
To calculate this you need to factor in gross weight of the trailer(which includes everything inside the trailer plus the fuel) and the second weight which you need to factor is the weight after the rear wheels of the towing vehicle till the front portion of the trailer(this includes the luggage in your trunk and in some situations may also include the weight of the contents in the front part of the trailer) and is popularly known as tongue weight.

So when this tongue weight becomes an appreciable part of the vehicle weight it can uproot the front end of the vehicle from the ground which may lead to loss of control and can result in a dangerous situation. All this can be simply averted by using a weight distribution hitch. So if you carry/transport trailers which have a appreciable weight when compared to the weight of the vehicle then weight distribution hitch is a must for you.

How to choose a weight distribution hitch?
For choosing a weight distribution hitch, you need to know the average gross weight as the tongue weight which you carry, then you need to buy a weight distribution hitch which has a capacity of +10% then the average you need. Please note then buying a distribution hitch which has a average weight distribution capacity, much greater then the one you require can often render it ineffective and result in some risky consequences.

Cost of weight distribution hitch:
There is a normal principle when it comes to buying these. The greater the weight you need to balance the greater is the cost. Also the cost depends on the type of technology used to balance the weight. Some of the widely used technologies are no-sway,round ball etc. A weight distribution hitch of 600-800lbs would normally cost you around $300-$500 depending on the brand and the technology used.

So if you usually tow trailers along with your vehicle it is better to have the safety of a weight distribution hitch.