Saffron is, per unit of weight, the most expensive thing you can buy in many supermarkets.
(That's my conjecture after noticing that the saffron in German supermarkets is about 10,000 to 30,000 EUR/kg, or 68,000 for the bio certified, hand picked, from a women-run enterprise:
https://shop.rewe.de/p/herbaria-safran-faeden/VJ7R7E3DA )
Any other contenders?
Ignoring the whopping (!) 10g bulk buy, cheapest I can get from Ocado in the UK is £4.68 for a gram. Vanilla pods start at £3 each (and no cheaper on Amazon other than in big bags, happened to be looking recently) and when you use one you use one; whereas the saffron probably won't be used all at once, again for a hand-wavy notion of 'similar sort of style/quantity' thing (despite obviously being totally different if one has vanilla and the other saffron..).