I have a LEGO minifigure and I'm trying to determine the value of it
At the moment (12/09/2018) there are a few other people offering that figure for sale on BrickLink, varying from £110 to £230 (actual asking prices are in various currencies listed on the results page).

More importantly however, is the Price Guide page - from there you can see that one figure apparently was bought this month for about £72, however in July one was purchased for around £108, while another was purchased for about £83.

From that, you could probably get a sale at around £100, but it might take a while to sell.