(価格が)青天井であるって英語でなんて言うの？
「服の価格は安いものは1000円未満から、高いものになれば青天井です」
のような文脈で使います。
回答
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the sky's the limit / sky-high / no limit
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Cheap clothes can cost as little as ￥1,000 but for expensive clothes, the sky's the limit.
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Cheap clothes start from about \1,000 but there's no limit to how expensive clothes can be.
「青天井である」＝ the sky's the limit / sky-high / no limit
「服の価格は安いものは1000円未満から、高いものになれば青天井です」＝ Cheap clothes can cost as little as ￥1,000 but for expensive clothes, the sky's the limit. / Cheap clothes start from about \1,000 but there's no limit to how expensive clothes can be.
例文：
cheap clothes ＝ 安い洋服
cost ＝ かかる
as little as ＝ ～ほど安い
expensive clothes ＝ 高い洋服
start from ＝ ～始まる
イギリス 
