Words can be many different parts of speech, depending on the context, the job they do in a sentence.
But "worth" is almost always used as either a noun or an adjective.
A prepositions "job" is to join nouns/pronouns/gerunds to the rest of a sentence. I can't think of any context in which "worth" would do the "job."
In the context of your example:
It is worth $1,000,000
"worth" is being used as a predicate adjective to modify the subject "It."
Worth:
"ADJECTIVE: 1. Equal in value to something specified: worth its weight in gold. 2. Deserving of; meriting: a proposal not worth consideration. 3. Having wealth or riches amounting to: a person worth millions."
Adjective
worth
Equal in value to; proper to be exchanged for.
My house now is worth double what I paid for it.
Deserving of.
I think you'll find my proposal worth your attention.
Valuable, worth while.
Making a fair equivalent of, repaying or compensating
This job is hardly worth the effort.
Usage notes
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". Fowler's Modern English Usage says, "the adjective worth requires what is most easily described as an object
P.S. Compound verbs:
1. a main verb plus (an) auxiliary (i.e. helping) verb(s): e.g. He has been reading
2. one subject doing two verbs that are joined by a conjunction:
e.g. He finished the paper and handed it in.
In the examples:
"it is valued at...." "it is composed of..." (you're right, ed) "valued" and "composed" are past participle adjectives being used in what's called "the stative passive."
P.P.S. It can't be a "predicate noun" since predicate nouns (which come after linking verbs such as "is") have to be interchangeable with the subject:
e.g. My brother is John. = John is my brother.
And "worth" isn't interchangeable with "It."