"It's the last week of cheap prices."
Translation:Es la última semana de precios baratos.
Always use "ser" when talking about the time things take place. Thinking of "temporary/permanent" is okay as a rule of thumb when deciding between ser/estar, but it can lead you astray. For example, the location of a building is pretty permanent but because it's location we use "estar" for it.
In a spanish course on languagetransfer.org they use characteristic vs state when talking about ser/estar. I find it more useful way of thinking. Also the core of the sentence is that it is a week. Everything else: last, prices are secondary and don't affect the choice of the verb. Consider this: "Estoy triste" vs "Soy una persona triste". First describes my state, second tells that I'm a person, though a sad one.
Limiting adjectives (adjetivos determinativos) are normally are placed in front of the noun. These include adjectives which indicate quantity [e.g., mucho(s), poco(s), cuanto(s), todo(s), dos, etc.], articles (el, la, un, una, etc.), unstressed possessives (mi, tu, su, etc.), demonstratives (este, ese, aquel, etc.), and moral qualifiers (buen, mal, etc. if not preceded by adverbial modifiers such as muy) and particularly the comparative/ superlative forms such as mejor, peor):