Gold is one thing the Fowls want. Their family motto is "Aurum Est Potestas," which means "Gold is Power". Artemis used to value gold more than anything before he became soft. He even asked for 1 ton of unmarked 24 carat gold as Holly's hostage fund and one metric ton of gold as payment for his C Cube when he intended to sell it to Jon Spiro in The Eternity Code. When Artemis asked for his consultant's fee when he agreed to help Holly in defeating Opal Koboi, he demanded 1 ton of gold from the hostage fund. He has said many times before that his father used to tell him the value of gold. He has often said that gold holds its value and that is why he likes it. He also said that to do successful stuff, you need leverage, and gold is leverage.
Once a human successfully separated gold from the Fairies, they get to keep it and no fairy can ever get it back, according to The Book.
The LEP have a hostage fund that has gold.