The first versions of each?

I tried poking around a bit and can't find accurate recent listings to give you a concrete answer. I saw one a Rarity sell with her accessories for $15. I know Applejack with the forelock is more sought after but she has a listing sitting on eBay, no accessories, for $10 or so.

If you want a starting point, I would say $15 or 20 for each mane 6 pony based off what I saw.

Luna and Celestia fetch a bit higher price, I would caution $25 starter for them but again, it's a bit tricky to find relevant listings. Which Celestia and Luna do you have? Tinsel, no tinsel?