From The Pan-American Post:
. . . commercial growers will be subject to a “variable fee,” which would ultimately be used to vary the price of the drug in pharmacies. . . . [A]dditional taxes may be added later . . .
Ultimately, the lack of built-in tax on marijuana sales — at least initially — is one of the strengths of the law. This provides important flexibility, allowing officials to accurately set the price to keep the drug competitive with the black market without worrying about additional taxes making it prohibitibely expensive for users. It’s also worth noting that this gives the Uruguayan government an advantage over authorities in Washington and Colorado (where the drug’s price will be set by state and local taxes as well as supply and demand).