Adjectives: cheap
Comparative: cheaper
Superlative: the cheapest
Other examples: clean, dark, light, short, tall, slow
late
later
latest
Other examples: large, loose, safe, strange, wise

thin
thinner
thinnest
Other examples: fat, sat, wet, red, big, hot, fit
sincere
more sincere
the most sincere
Other examples: boring, careful, modern, comfortable
happy
happier
the happiest
Other examples: dirty, friendly, funny, noisy, tidy, silly
stupid
stupider / more stupid
the stupidest / the most stupid
Other examples: clever, common, polite, gentle
quietly
more quietly
the most quietly
NOTE: A limited number have comparative and superlative forms with –er and –est:
fast
faster
the fastest
Other examples: early, hard, late, long, soon
good better the best
bad worse the worst
far farther the farthest
far farther the furthest
old older the oldest
old elder the eldest
well better the best
badly worse the worst
little less the least
much more the most
little less the least
few fewer the fewest
much more the most
many more the most