cheaper. Follow this rule: 1 syllable: -er and -est bigger, smaller, faster, slower, cheaper EXCEPTION: more fun 3 or more syllables: more and most more intelligent, more beautiful, more desultory 2 syllables: Look at the end of the word. If it ends in -y and isn't an adverb, use -ier and -iest uglier, shinier, happiest if it is an adverb, use more and most more quickly, more slowly If it ends in anything except -y, use more and most more alive, more upset, more wooden

Cheaper would be correct

In addition to what has been said by the others, note that you may see these two used correctly words together, but not as a collocation: “There are more cheap brands in store A than in store B.” Here, “more” refers to “brands”, not to “cheap”. The meaning is “more brands that are cheap”.