The life of a cattle grid will depend on 3 factors. Firstly has it been designed and made correctly. Secondly has it been adequately protected from corrosion and thirdly is it maintained. There are 3 or 4 firms which make units to an agreed standard – even based on British Standard occasionally!
There are any number of small fabricators who will have a go at making a unit with varying degrees of success. Using a local supplier is good for the local economy. It will save on significant delivery charges and is easy to do. If however it is incorrectly designed and/or made and it fails (and they do) the costs will outstrip any saving on purchase cost.
There are really only 2 options to protect a cattle grid from corrosion. The right way and the wrong way.
The right way is to galvanize it. The galvanizing process strips off the Mill Scale and metallurgically bonds the zinc to the steel. This is not a coating it becomes part of the grid.
The wrong way is to paint it. Painting, no matter how well applied will wear off and corrosion will begin. Galvanising gets inside and protects the tubes from the inside. Paining does not.