This is one of the rare books where the title isn't mentioned in the book aside from the cover page. We're talking about the original title, of course, which is The Price of Salt, not the re-release title and the title of the film, Carol. That one is self-explanatory.
The Price of Salt is a lot more ambiguous. Therese mentions salt a handful of times. Richard picks up a saltshaker. Therese and Carol look at Salt Lake City on a map. After Carol leaves her, Therese mourns, "How would the world come back to life? How would its salt come back?" (22.2). But later, when Dannie shows up, she says he brings "a little salt" (22.73) to her life, leading us to wonder if she'll choose Dannie over Carol. It seems romance is the seasoning Therese's life needs.
So salt is like a little spice of life, something Therese lacks in her boring department store job at the beginning of the book. But what about the "price" part? Therese does often worry about the literal price of things, paying the bill when Carol isn't looking and refusing loans from her.
But perhaps we shouldn't take things literally in this case. Maybe the "salt" is simply the spice of life, which Carol and Therese find in a taboo relationship with one another. And the price? It's the price they have to pay to society, where their same-sex relationship is forbidden. In this case, Carol is the one who pays the price—she loses her family to be with Therese.