The underlying value of gold bullion coins is calculated by multiplying the coin’s gold content by the current price of gold in the buyer’s chosen currency. While a buyer (or seller) must then allow for retailer mark-ups (or dealer discounts) totaling 3-8% or more, this means that how much a gold coin is worth changes with the live ‘spot price’ of gold, which you can track using BullionVault’s live gold price chart.
The value of ‘numismatic’ gold coins has less direct relationship with the wholesale bullion market. Their worth is dictated by the coin's rarity, its collectability, and also buyer sentiment, rather than by their pure gold content. This means it may be less prone to fluctuation short term.