This guide assumes that the furbies in question are working and not NIB (new in box), and that the buyer is buying in the United States.
For non-working furbies, halve the price.
For NIB furbies, double the price and subtract $5-$20. (This does not apply to angel, hi-c, kid cuisine, gen. 8, and prototype furbies, since they are rare enough that they will go for the highest prices regardless of whether or not they are NIB.)
This guide does not factor in the cost of shipping.
Furbies that come from the JT factory (for example, Kid Cuisine, Jester, and Tropical furbies) are often broken even while NIB due to poor training of employees of the factory. Many JT furbies are even missing internal parts and cannot be fixed even after manually spinning the motor.
Also, furbies are old and prone to breaking over the years. Many sellers on eBay and other sites do not list whether a furby is broken or not, or even test them at all. It's a good idea to ask sellers if the furby you're looking to buy is working if the condition is not included in the listing.