There are many companies (some on our website) who offer tours of the Tulip Festival. They might have sold out their capacity on a given date. The Tulip Festival itself does not sell out.
RoozenGaarde and Tulip Town have weekday admission of $7 per person, per garden with children 5 and under free. The weekend admission price is $10 per person, per garden, with children 5 and under free. Tulip Town offers an additional weekend discount for children 6-12 for $5; RoozenGaarde offers a $1 off the price of admisson for military with ID.