Tickets cost 20 dollars or so.
The sentence is from a dictionary. (Source)
Why there is no "the" article if it is definitely said about particular tickets?
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Express a general idea with a plural countable noun:
Tickets to the opera can be very expensive.
Tickets go on sale at noon.
Apples are good for you.
Rich people are a pain in the neck.
Tom: Hey John, what do tickets cost? [non-specific, but they already know what they are talking about]
John: Tickets cost 20 dollars.
Tom: What do the tickets for that school play cost? John: The tickets for that play are $10.00
It is not definitely said about particular tickets.
Tickets cost 20 dollars. There are no particular tickets that have been mentioned, so why assume that.
If only one ticket was mentioned you would say
A ticket costs 20 dollars.
Here it is explicit. The article is indefinite.
It would be grammatical to say
I have some tickets and some vouchers. The tickets cost about $20.
Because now the tickets are the particular tickets mentioned in the first sentence.