Learn German: Sie machen mir einen guten Preis für das Bett. - They are giving me a good price for the bed.

Preis is a masculine noun. Here, it is the direct object of the sentence, so it is in the accusative case. When a masculine noun in the accusative comes after an indefinite article like einen, the adjective gets the ending -en. That is why we say einen guten Preis.

Why does the translation say "giving" when the German word is machen?

While machen usually translates to "to make" or "to do", German uses the set phrase einen Preis machen to mean offering or giving someone a price. If you used geben (to give) here, it would sound unnatural.

Why is it mir and not mich?

mir is the indirect object, which takes the dative case. The direct object being offered is the price (accusative). The person receiving that price is the indirect object, so we use the dative pronoun mir instead of the accusative mich.

Does the word für affect the case of das Bett?

Yes! The preposition für always triggers the accusative case. Because Bett is a neuter noun, its accusative form looks exactly the same as its basic dictionary form, so it stays das Bett.