"Is this free?"
是 + adjective + 的 is the sentence structure for adjectives that do not have a scale or range to them.
If you look adjectives such as 好(good), in English, you can say "I am good", but you can also say "I am SLIGHTLY good", "I am VERY good". You can essentially have varying degrees of good. This is why these types of adjectives are paired with degree indicators such as 很好， 非常好， 最好 etc.
However if you look at the word "free", you do not say "this is SLIGHTLY free" or "this is VERY free", in this context, free means no cost, and free is just free. For these types of adjectives where the degree cannot be indicated, you must use 是 。。。的.