There is a very good reason why metallic reel seats are not favoured for salt-water fishing and it has very little to do with costs for design and manufacture, robustness or weight.
Salt-water is one of the best electrolytes naturally available and will cause Galvanic corrosion (which is an electrochemical process) between the reel-seat and reel, unless adequate cathodic protection or insulation is used. Meaning that when you assemble two dissimilar metals together and expose them to salt-water, corrosion will automatically and inevitably occur producing further damaging acids along the way. The rate of the corrosion will be determined primarily by the metallic elements selected. The majority of modern fishing reels are actually manufactured using Aerospace grade Aluminium to prevent corrosion from oxidation. Some of the more high end reels use advanced composites to prevent this. A Titanium reel-seat with a typical modern cast /machined Aluminium reel, although very strong and light would pose a very high corrosion risk and cause rapid corrosion because the materials are so dissimilar when it comes to their galvanic properties. Even if the reel-seat / reel where plated or coated, the continued use and assembly would soon wear / chip the coating away due to metal to metal contact and corrosion would set in.
Century, Abu and the like are well aware of this principle, hence the use of plastics to prevent the situation from occurring.
Generally, using metallic reels / reel-seats for salt-water fishing is a guaranteed way to prematurely wear or age your expensive gear!...Just my tuppence worth.
Regards
Si