I think you're asking why "extracted" and "was" are both in the first part of the sentence together: "Extracted" isn't actually a verb here; it's being used as an adjective describing the colour. When used this way, it's called a "past participle", if you want to read more on this. In fact, "The purple colour extracted from a snail" is the subject of the sentence--we haven't reached a verb yet. It's easier to see when you replace those words (the subject) with "it": "Indeed, it was once so costly that..." When written this way, you can see that there is really only one verb: "was".

[moved below to "Answer"]

You can understand was once as used to be.