Full soulstones come those days from Legacy tokens, which are for sale on the official store for $9.99, so they should be worth 10m-12m. On most shard you buy them for 12m (or 14m-15m in Luna). Now with Europa lacking xshard traders you'll have a hard time finding anything coming from the official store, be it soulstones, earrings of protection, forged artifacts, etc, at least at a non-ridiculous price.
I've been looking for those things myself since weeks and I'm now considering getting a xfer token and buying them on a "normal" shard.