In general this is referred to as 'noun ellipsis'. https://en.wikipedia.org/wiki/Noun_ellipsis
Ellipsis refers to omission generally.
An example of noun ellipsis that 'I like the blue shirt, but I don't like the pink'.
According to Christine Gunther at https://doi.org/10.1017/S1360674311000049 the most common use is with colours.
However there are other types that are commonly used, such as 'the Bruneian dollar has the same value as the Singaporean', where the two adjectives are related.
In your case it can be seen that the adjective 'free' is related to the phrase 'best deal'.
Gunther identifies antonymic relationships (e.g., ' A narrow path led between the two waters, the wild and the domesticated.') between adjectives as well as superlatives (e.g., I went up that skyscraper in Boston, but the tallest is in Chicago) as typical.
In this case, the sentence is colloquial, and in some sense superlative, in that 'free' is the best possible deal.