Ohhhh a toughie, i'd say the going rate is about £50 upwards for a goodun, thats no bolster wear, broken frames, missing badges or saggy bottoms anything less than this is gonna be worth about £20 each imo, no better than a set of worn recaros really. Really mint seats have been known to go for silly money on fleabay

You are more likely to pay more for a good drivers seat than a passenger one, as the drivers ones tend to wear more than the passenger side (for obvious reasons)

hope that helps