In general, if the miniset was not available for gold, the value you get for 15$ is quite good. 15$ for a bundle that includes 4 specific legendaries, 1 epic (x2) and some more rares and commons is pretty good value if you are looking to spend some money in the game and definitely better than all the alternatives you have at the moment.
However the same can be said for gold. Saving the gold to buy 20 packs at some future date is not better than buying the miniset with it in terms of expected value.
Finally, there is a more practical aspect. The powerlevel of the miniset is quite underwhelming and most cards are unlikely to see any play. Therefore it might be a questionable investment, whether with gold or money