Why is 'worth' adjective?

朋友，你的觀察力真是非常好，若你查考牛津第六版，在worth條下，它就有以下的敘述：
adj.〔usually used like a preposition常發揮介詞的作用〕
而LONGMAN contemporary English更直接將它看作Preposition.
英文有時真是很難解釋，但最要近的是，我們曉得怎樣正確用它，相信，你對這字的運用，完全沒有問題。
希望幫到你！

2008-04-11 14:17:51 補充：
其實還有很多的predicative adjective，需要complement，如：

afraid：He is araid to do it
fond：They are fond of fer

有些 adjective是需要用prepositional phrase作complement(Adjective complementation by prepositional phrase)，如：
Joan if fond of them
They are conscious of their responsibility
We are ben on a vacaton in Mexico

2008-04-11 14:17:57 補充：
這些有：intent on, reliant on, averse to, liable to, subject to, inclined to, familiar with等

2008-04-11 14:21:16 補充：
有時adjective又要用finite clause作complement（Adjective complementation by finite clause)
如：
I am sure that he is here now
I was angry that he should ignore me
I was admant that he (should) be appointed
I am not clear where she went.

2008-04-11 14:24:53 補充：
有些時候，有些adjective又需要to-infinitive clauses作為complement(Adjective complementation by to-infinitive clauses)，如：

Bob is slow to react.
Bob is furious to hear it
Bob is hesitant to agree with you
Bob is hard to convince.

總括來說，其實adjective要有complement都是常見的事

2008-04-11 14:25:52 補充：
打錯字！對不起！
英文有時真是很難解釋，但最要緊的是，我們曉得怎樣正確用它，相信，你對這字的運用，完全沒有問題。

2008-04-11 14:27:25 補充：
又打錯字：
afraid：He is afraid to do it
fond：They are fond of her

We are bent on a vacaton in Mexico

2008-04-11 14:28:24 補充：
I was adamant that he (should) be appointed

2008-04-13 21:00:13 補充：
我想補充少少，
think'Although the dress is expensive, I think it worth (it)這句，並文法上是完全正確，但這句的worth仍然是adjective.

這是think的一個特別的用，think可有這個形式

think + noun + adjective，如：
I think it highly unlikely that I will get the job.

2008-04-13 21:01:54 補充：
而I think it highly unlikely that＝I think that it is highly unlikely that...

所以I think it worth (it)＝I think that it is worth

所以，希望朋友你不要誤會，這是錯誤句子！

First of all worth is not only an adjective, it can also be used as noun.

When we use worth as an adjective, we use it as a predicative adjective, that is = determinating adjective, this adjective has a determinating nature.

Your quoted sentence - something does not worth this price - THIS PRICE is a noun clause. So grammetically this is correct.

Many words cannot stand alone or else they have no meaning, however we cannot call all those words as preposition.

When it is used as noun, the meaning is value :
E.g. Although the dress is expensive, I think it worth.

Again as I mentioned on above, we cannot treat worth as an incomplete adjective, it is a predicative adjective.