le 了 at the end of a sentence with a stative verb predicate.

1. 
   Tiānqi lěng le.
   天气冷了。
   The weather has become cold.


This sentence implies that it wasn't cold before and that the weather has changed. Compare it with the following sentence which does not have the change-of-state le 了.


2. 
   Tiānqi hěn lěng.
   天气很冷。
   The weather is cold.


This sentence simply states the fact that the weather is cold.


3. 
   Dōngxi dōu guì le.
   东西都贵了。
   Things have all become expensive.


This sentence implies that things were not expensive before and that the price has changed. Compare it with the following sentence which does not have the change-of-state le 了.


4. 
   Dōngxi dōu hěn guì.
   东西都很贵。
   Things are all expensive.


This sentence simply states the fact that things are expensive.

In a sentence that has a stative verb as the predicate, the adverb hěn 很 is used unless a comparison is to be made. If the sentence takes a change-of-state le 了, hěn 很 is eliminated. This is because a change in situation often implies a comparison with the past. For example, when one says that the weather has become cold, it is a result of comparison with an earlier situation.