Even the youngest Bs are now nearly 8 years old. At that kind of age, the value of any car depends on things like condition, mileage, specification, colour & service history. Also factor in the time of year - Bs always fetch more in the spring/summer. Most Fiats are only worth pennies at that kind of age, so the fact that Bs still fetch decent money is a testament to their desirability and relative scarcity.
The best guide, as you suggest, is to look in the classifieds (pistonheads, autotrader etc), although this won't tell you what price they're actually selling for (ebay's useful for that, as you can search for completed auctions).