My only thought would be a difference in the quality of the stainless steel being used in the construction. There are several different grades of stainless steel available and not all are really suitable for use in the sea long term.
Marine grade stainless steel is much more expensive (several times the cost) of ordinary stainless so this could easily account for a big price difference between two similar looking products. If you are able to strip down and thoroughly rinse and dry something after each use it won't matter so much, otherwise the 'wrong' grade of stainless will degrade and rust fairly quickly in sea water. It doesn't say which sort of stainless is used in this mount, just states - "Made from Strong, Light Weight, and Corrosion Resistant Stainless Steel" which could mean any grade, and not necessarily marine quality.
I have bought similar things before and been very happy, but conversely I have bought some and found the materials to be inferior. It depends how much that matters to you and whether you are religious about cleaning and maintaining after use. - Have fun - Phil