たかいです and やすいです are fixed reactions to a price — "pricey!" and "cheap!".
JLPTN5
たかいです／やすいです
Use たかいです and やすいです unchanged when you react to a price — just the adjective plus です. Treat them as set phrases: nothing here changes form, and the rest of the い-adjective system (negation, past, describing a noun) is a separate topic.
hanamaru
