There are different factors to consider when trying to determine the value of a specific figure and in this post Im going to cover them.
Once you know what figure it is that you have, you'll find a broad range of prices on any site you check and that can be attributed to the condition. The ideal condition is not just new but also never assembled, preferably sealed in a ziplock with no exposure to sunlight. These Minifigures are worth the most on the after market and that is why Imperial Brickz Prices are above average. Assembled Minifigures that have only been displayed can hold similar value however, even displaying them exposes them to air and light which can cause the plastic to yellow and crack lowering the value of the figure. Used or "played with" Minifigures will be worth the least as the plastic will show wear around the feet and under the arms. Legs and arms on Minifigures are also known to crack, some colors like reddish brown and dark red especially.
The second factor to consider would be location. Some sellers may have really low prices but could be located over seas so exchange rates and shipping may play to the buyers disadvantage. Filter out those sellers so you can accurately find the value according to the supply/demand of the Minifigure in your country.
Finally the version of Minifigure you have. After almost 25 years of LEGO Star Wars some Minifigures have 10+ variants and these can greatly affect the value. Be sure to check the colors of the pieces and the colors of the prints. Some figures with arm and leg printing are worth 10 to 20 times more than those that don't have them.
Now all you have to do is search the internet and try to find the best price possible regardless if you're buying or selling, just be sure to consider these factors to better understand the market.