Eliot's right: "cheap" has a long vowel sound. We only add an extra consonant if the vowel is also short. (Without it, "biger" "fater" and "sader" all indicate long vowel sounds.)
Israelphoenix has covered the second part of the question well, but I have only this to add: stop using "wanna" in your writing. Native English speakers who write like that are considered illiterate.