Learn Portuguese (Portugal): Estes sapatos são muito baratos.

We use estes (these) for things that are close to the speaker, like shoes you are currently holding or pointing right at. If the shoes were closer to the person you are talking to, you would use esses (those). Both are masculine plural to match sapatos.

Is estes the plural of isto?

No, estes is the plural of este (this). We use este and estes right before specific masculine nouns, like sapatos. Isto means a general "this thing" (as in "What is this?") and does not have a plural form.

Why is the verb são used here instead of estão?

São comes from the verb ser, which we use for permanent or defining characteristics. Being cheap (baratos) is seen as a built-in quality of these shoes. If you wanted to say they are temporarily cheap because of a sale, you could use estão (from the verb estar).

Why does baratos have an -s at the end, but muito does not?