"the price for an item is too high" - not a noun phrase. Actually, this is a clause, as it has a predicate of its own. "a bottle of water while strolling" - not a noun phrase. "A bottle of water" would be a noun phrase (a phrase that has a noun as a headword), but "while strolling" is an adverbial phrase that describes the verb "buy". "The same brand of bottled water" - noun phrase. The headword is "brand", and all the other words cling to it and provide additional explanation. "a pair of athletic shoes" - noun phrase. The headword is "pair".
"A noun clause is a dependent clause that acts as a noun. Noun clauses begin with words such as how, that, what, whatever, when, where, whether, which, whichever, who, whoever, whom, whomever, and why. Noun clauses can act as subjects, direct objects, indirect objects, predicate nominatives, or objects of a preposition."
