There will be salt readily available at almost every restaurant you visit.
A salt shaker sitting on the table.
Packets made available at a condiment station.
Perhaps, in higher-end establishments, a member of the wait staff who will walk over to your table with a salt grinder and season your food to your liking.
And in all cases, there's no charge for the salt. You can use as much or as little as you like, and you pay... nothing.
The restaurants pay for the salt, of course, but then they willingly accept a loss on that purchase by providing it to you for free.
Because having salt readily available is a really easy, inexpensive way for restaurants to increase the chances that customers will enjoy the food they've been served.*
The cost of the salt is completely negligible relative to the extra value it offers, and the increased customer satisfaction that results.
Another example: resealable packages for grocery items cost consumer-packaged-goods manufacturers marginally more than packages that can't be opened and closed repeatedly, but those resealable packages are significantly more convenient for consumers.
For CPG manufacturers, more convenient packaging is the "salt" they make available for their customers. Are those extra costs worth it? Absolutely.
Think about what "salt" you can offer for your customers: what are you able to provide, at a relatively low cost to you, that represents a significant benefit to them?
Getting a return on your investment is important, of course. And you need to do that math.
But as you do your calculations, don't forget about what a truly satisfied customer is worth.
* Another benefit is that it also allows the restaurant to make one standardized dish that can be enjoyed by many different customers with a wide range of flavour preferences. For example, I very rarely salt my food and my wife salts almost everything... but that free salt lets us go to a restaurant and order the same dish, then customize it to our individual tastes. That's another clear benefit, but not the one I wanted to focus on for this post.
P.S. The idea was sparked by this blog post. The only similarity between my post and Seth Godin's is that they both use salt to illustrate a broader point. Still, credit where credit is due.