I suppose it is a question of supply and demand!
LP1 weights are very rare - as you say - and anyone can ask whatever price they want if they have one to sell but what it actually sells for may be another matter so why not make an offer and see how you get on?
If the weight really does sell for £50 then somebody somewhere obviously thought it was worth it.
I have an LP1 pistol that also has these weights - if they really do sell for as much as £50 each (?) it would be tempting to sell one of them but to be honest I would rather keep the pistol complete with the weights as I also think it would be very hard to replace them again.
If I remember correctly there are two sizes of weight offered as options for the LP1. I have never used either of mine as I like the balance of the pistol the way it is without them .
Maybe I should dig them out and keep them in the safe from now onwards - I never realised they were that valuable