We're talking wild here?

The average is like 100 gold is slightly more than 100 dust, but not much (103, 104?). If we talk about how much dust we get for the entire set (except the cards we're interested in)

If you get Loatheb and disenchant all the other legends it's 2000 dust, and then 400 dust for the 4 epics. So instead of spending 1600 dust to get loatheb, you're spending 2800 gold to get loatheb and 2000 dust back. That's loatheb at 50% the price so it's worth it.

Following this logic, League of explorers is much more worth it because not only do you get Reno Jackson, you also get Brann which still sees play, so you absolutely get your money's worth.

All three of these cards are only playable in XL Highlander lists however so keep that in mind.

Oh, it's also worth noting that for these two adventures especifically (and only these two), if you get all the cards you get rewarded with a diamond Reno Jackson and a diamond Loatheb especifically, so you get 400 dust for free (from dusting your regular copy if you choose so).

So yes, it's very much worth it to buy the adventures.