le 了 at the end of a shì 是 sentence.

1. 
   Xiànzài shì xiàtiān le, cài dōu piányi le.
   现在是夏天了，菜都便宜了。
   It's summer now, (so) vegetables have become inexpensive.


This sentence implies that vegetables were expensive before. Compare it with the following sentence which does not have the change-of-state le 了.


2. 
   Xiànzài shì xiàtiān, cài dōu hěn piányi.
   现在是夏天，菜都很便宜。
   It's summer now, vegetables are inexpensive.


This sentence simply states the fact that the time is summer and that vegetables are inexpensive. It does not imply any change.


3. 
   Tā shì lǎoshī le.
   他是老师了。
   He is a teacher now. He has become a teacher now.


This sentence shows that 'he' has come to a new stage of his life or a change in identity. Compare it with the following sentence which does not have the change-of-state le 了.


4. 
   Tā shì lǎoshī.
   他是老师。
   He's a teacher.


This sentence simply announces 'his' identity as a teacher.