The word costliness is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 15
Highest Scoring Words With Friends™ Play In The Letters costliness
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Seven Letter Word Alert: (31 words)
cession, clients, closest, closets, consist, cosiest, cosines, enlists, incests, insects, insoles, lection, lesions, lessons, lioness, lionets, listens, nosiest, notices, oecists, osseins, ossicle, section, session, silents, sinless, sonless, stencil, tinsels, tocsins, tonsils