Is it worthy to publish an extended abstract in a conference?
The aspect of what an extended abstract is differs from one field to another. Refer this page to get a better idea on what does it mean in your field:

What is an extended abstract?

For example, in CVPR, 
extended abstracts are handed out to the attendees before the start of the conference. They are much more handy than a huge proceedings book as each article is much smaller than its full version and includes all relevant details at a glance. It serves as a means to attract attendees attention especially when there are multiple venues occurring in the same time.

However, whether the amount you pay for it worth it or not depends on various factors, such as the field, the tier of the conference and so on. Sometimes in top tier conferences, there may be funding agencies who might be attending the conference. In such cases extended abstract would be a pretty valuable. But in any case, this decision would be better sought out by consulting it with your advisor or faculty members of your department as @ff524 suggested.