Why does everything cost more with saltwater

Probably has to due with the fact that the equipment that is required is built to be way more reliable, have to do with the corrosion aspect of salt in water, and probably that there tend to be more pieces involved with a saltwater filter vs. a freshwater filter to handle other aspects.

Have you tried looking on eBay or OfferUp or even on here for some type of used tanks in your area or equipment?

salt tends to eat at metal. So things need to have a special type of metal in order to not break down super early

Higher need for robust materials and precision equipment.

As mentioned, salt tends to break things down quicker, due to it being corrosive in nature. The higher the salt content, the harder it is to maintain good equipment. Wouldn't want your equipment breaking down fast, now would you? Plus having to spend the money for good quality equipment is always a plus. The better quality the tank and equipment is, the longer you could go between maintenance fixes that would require replacing parts.

I noticed that salt equipment is higher too. The initial set up of a tank requires more things too that are different from a fresh water tank. I was told that the fish are harder to take care of but I've never actually confirmed that! So maybe all that combined allows company to charge more?