It's usually limited by the individual dealer, and the limit varies depending on what the item is. A couple of my friends actually pay for their trip to the convention by buying as much limited stuff as they can and eBaying it.
I think it depends. Hasbro currently has some of the DJ Pon-3 and Princess Twilight Sparkle exclusives on hasbrotoyshop right now for $50 each(which Im pretty sure is how much they went for at the con). Its way better than the eBay prices. But if you wanna get them you should probably act fast. Last year the Derpy figure sold out on Hasbro's website in a couple hours.
Thank you! I just got DJ Pon 3 which is the one my dd wants! Last year I wasn't fast and missed Derpy, got that on eBay!