The question "Not worth the paper it's printed on" - wrong meaning? got me thinking about what part of speech, or lexical class, the word 'worth' takes?
A comment in "Is it worth it?" vs. "Does it worth it?" advises to treat 'worth' as an adjective, but I'm not sure that's right. 'Worthy' is an adjective, but that's not the same as 'worth'.
In the title question you could replace 'worth' with the phrase 'of comparable value to'; what part of speech would that phrase be considered? Are the word and the phrase in the same lexical class?
Just to throw another option out, after a stimulating discourse under the answer provided by @Henry:
Wiktionary mentions that used in the context of this question, 'worth' is considered an adjective, but it also notes that
The modern adjectival senses of worth compare two noun phrases, prompting some sources to classify the word as a preposition. Most, however, list it an adjective, some with notes like "governing a noun with prepositional force."
It also notes that
Joan Maling (1983) shows that worth is best analyzed as a preposition rather than an adjective.
If viewed as a preposition, then it is easy to replace 'worth' with another preposition of somewhat equal meaning, such as "Is this apple about $3", "Is this apple under $3", or "Is this apple over $3".
Adjective, infinitive, or preposition? ...or is this a rare case of an old language usage that straddles multiple lexical classes in modern language?