1.) Networking. This, to me, refers to both putting your handshake on your peers and other industry pros, and (if published) getting yourself in front of readers/fans. Admittedly, I have been a very shy just want to stay in my room person in the past. (KAK, I saw your post yesterday. I'm guilty of it.) But I've grown. Bartending for any length of time will chase the shy right out of you. But there are sooo many cool awesome people out there in this industry. Go find them! (But don't stalk them.)
2.) Panels. There are usually multiple tracks going, and something for everyone throughout the span of the convention. You can learn tips and tricks, you can hear stories that inspire, you can go listen to the real personality behind the books you've enjoyed--and perhaps discover a few personalities that make you want to read their stuff. Pay attention. What made you want to read their stuff? You're going to want to have that same effect on others someday, right?
3.) Workshops. I've attended many. I've given a few. Usually, the workshops a con offers are very much worth the time, cost and effort. You have someone who has proven themselves at the head of the room, talking in depth about a subject you elected to learn more about. How can you not get inspired here? And, those likeminded folks around you are more networking opportunities....
4.) Friends. I've made a few friends via conventions. As I mentioned above, those like-minded folks in a workshop with me. That can turn into a writers group, beta readers, a group blog, who knows. Going to conventions is about being serious and exposing yourself and your work to someone other than your mom.
5.) Persona/Image/Brand. During this exposure to the industry and the people who populate it, you will see/hear/meet people with personalities from one end of the spectrum to the other. Be true to yourself, always, but remember being an author does involve a public image and a brand. You'll see what draws people in and what is just off-putting, and when it's your turn to shine, hopefully you'll make good choices about how to let your best qualities shine brightest.
In the end, conventions, in my opinion, are always worth it. I come away inspired and 'jazzed' up, ready to write. You can't put a price tag on that kind of inspiration.