Here in Australia for some reason Thrones of Blood is a different price than Steam.

All the other prices are the exact same between the systems, but for some reason Thrones of Blood is more expensive on GOG.

Steam AUS$ 28.95
GOG AUS$ 31.99

But all other DLC and the base game are exactly the same price. Is this a mistake for Thrones of Blood?

If it's not a sale example, probably. But steam also demands that no one have a lower price than them.

That doesn't mean other people can't have the same price as them. Just that they may have larger sales.

If steam doesn't have workshop for a game, I'd still consider it worthwhile to spend a few dollars more to get a permanent gog installer.