There are a couple of reasons why rewetting drops are more expensive.

They usually _do_ include something besides salt. Often it's methylcellulose
or polyvinyl alcohol, which primarily increase viscosity. There are other
ingredients, but for each one the manufacturer is required to demonstrate
safety, effectiveness, shelf life, and resistance to bacteria. ReNu recently
got burned because a new ingredient eventually promoted a weird fungus that
nobody required testing for.

Plain old salt water is available as "unpreserved" tear supplements, and the
price looks even more shocking except for the fact it's guaranteed sterile
when opened, and guaranteed pure, and comes in a convenient package that
won't hurt you or turn it to acid on your dashboard in the summer. Wetting
drops are a pretty good deal considering the incidence of contamination and
infection remains low if you follow the instructions.

There are plenty of more shocking stories - look at the price and patent
history on the "Tobradex" combination. Or my Byetta - it's $33,000 per
*gram*.

-MT,OD