Learn Arabic (Modern Standard): سعر هذه الطاولة سبعون دولارا أو ثمانون دولارا. - The price of this table is seventy dollars or eighty dollars.

In Arabic, the 'tens' numbers (20-90) change their endings depending on their role in the sentence. Because they are the main information being stated about the subject here (the predicate), they take the nominative case ending ون (-un). If they came after a preposition or were the object of a verb, they would end in ين (-in).

Why is the word for dollars written as دولاراً (singular) instead of plural?

This is the special rule for numbers from 11 to 99! After these numbers, the counted noun must be singular. Even though in English we say 'seventy dollars' (plural), Arabic requires the singular equivalent of 'seventy dollar'.

What is the extra alif at the end of دولاراً?

That extra alif holds the 'an' sound (double fatha) for the accusative case. For numbers 11-99, the counted noun must be in the accusative case. Unless a word ends in a taa marbuta (ة), this case requires a silent alif added to the end of the word to support the double fatha.

Why do we use the feminine 'this' (هذه) if 'price' (سعر) is a masculine word?

Because هذه belongs to the table (الطاولة), not the price (سعر). The phrase is 'the price of this table'. Since الطاولة is feminine (indicated by the taa marbuta at the end), we use the feminine هذه to point to it.