"cost extra 50 dollars" or "cost an extra 50 dollars"?

Question

Tell me please I need to use the indefinite article in the following sentence.

That phone costs 200 dollars, and that one will cost you (an) extra 50 dollars.

I think because of the adjective extra I need to use the article, but I am not sure. If it is really needed then why?

Answer 1

The indefinite article is used because $50 from somewhere is needed.
If the $50 dollars was specific, you would use "the"

That phone costs $200, and that one will cost you an extra $50.
You should pay for it with the $50 you have in your savings account.

The second sentence also implies you only have $50 in your savings account.

You should pay for the extra $50 from your savings account.

Refers to the specific "extra $50".

Answer 2

Something can cost you.

Something can cost (you) extra. ^{extra = more}

Something can cost (you) twenty dollars|pounds extra. ^{extra = more}

Something can cost (you) an extra ^{extra = additional} twenty dollars|pounds.