This is a very difficult question to answer due to the wide scope of it. Asking what the most expensive of any category throughout all of history is very difficult to answer because there are so many factors involved. You have to understand that what makes something expensive is not only how difficult it is to produce but also how abundant the materials are in a given area. For example Silk has widely been regarded as a very expensive and finely crafted material exported from china throughout the history of Imperial china. Many would not consider some other textiles(for example cotton) as being of equal quality. Therefore it would be safe to assume that silk would be much more expensive than cotton. That being said there were periods of time in which cotton was far more expensive than silk within China because due to the environment, and techniques in China at the time cotton was more difficult to produce at equal quantities to Silk. Having said that I can pose an answer to you question in regards to a specific area. Throughout Antiquity, in the Mediterranean region the Tyrian Purple dye, also known as Royal Purple, was incredibly prized and expensive. I believed it is this Dye that Raidgroupcomps is referring to. It was incredibly difficult to obtain because it require thousands of a specific type of shell to create very small quantities. And unlike other dyes weathering actually enhance the color rather than caused it to fade. I hope this was helpful!

Reiterating the words of a colleague who focuses on Roman history:

Purple. Purple dye came from scraping the inside of oysters. It took something like 1000 oysters for one drop of purple dye. It was a sign of wealth to wear a toga with purple stripe on the sleeve. And there was some social faux pas where a person would get fake dye and paint his purple stripe, but it would run.

Julius Caesar had an entire toga dyed purple to flaunt his wealth and power to the Senate. I asked my colleague for a modern comparison, and he said that dying that one toga was the equivalent cost of the Palace of Versailles.

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