Structure
Noun + だけしか + ない
Details
Standard
About だけしか
だけしか is a combination of two adverbial particles, だけ 'only', and しか 'nothing but'. This structure primarily follows nouns, before part (B) of the sentence will have a corresponding ない based word.
Despite all of these possibilities, it should be noted that だけしか~ない is an expression that appears with negative verbs, far more than any of the other combinations.
Fun Fact
だけしか may appear as though it is saying 'only' twice, but each particle has its own specific nuance. These nuances are as follows:
だけ - Removes one thing (from within a group), and presents it as the 'only' thing that fits a certain criteria.
しか - Highlights something that exists by itself as not having any alternatives.
Due to this, each meaning compounds together, with だけ saying 'just this', while しか adds 'and only this!'. Reversing the order and putting しか before だけ would not work, as the sequence of selection needs to be logical.
Synonyms
Related
Vocab Coverage
All Bunpro Vocab that appears on this item.
Examples
--:--
この時間、医師は一人だけしかいません。
At this time of day, there is only one doctor present.
椅子はここにあるものだけしかありません。
The chairs here are the only chairs.
私には従兄弟が一人だけしかいません。
I only have one cousin.
僕は日本語だけしかできません。
I can only speak Japanese.
現金は今1000円だけしかありません。
I only have 1000 yen in cash.
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How to use しか & だけ & だけしか
Maggie Sensei
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だけしか – Grammar Discussion
Most Recent Replies (18 in total)

Durdool
Thank you. Indeed, that was also my understanding; the だけ is there just for extra emphasis and the meaning of the sentence should not change meaning if you just deleted だけ from だけしか. My initial understanding was exactly as yours, i.e., “only this cup costs 500 yen, why is that?”
The same inconsistency appears in another one of those example sentences.

Einlar
But that example sentence has “下手”, not “上手”. So it would translate to “In this group, Takeshi is the only one that is bad at it”, which is rendered with “isn’t good” to make it softer I think (but this confused me a lot at the start).
I agree for the sentence about the cup:

Durdool
You are very right, Einlar. I somehow read 下手 as 上手 and that, of course, flipped the meaning. Therefore, going back to my previous comment, the sentence without だけ is “この中ではタケシしか下手じゃない,” meaning “everyone, except for Takeshi, is not bad,” meaning that Takeshi is the one that is bad (or not good, as the example sentence). Thanks for catching that.
As a side note, I made this comment after reading the previous comments (above) of camytang and BambooCake of July and December 2022, where they discuss the same sentence but the word is 上手, not 下手. Perhaps the sentence has been corrected between then and now.