Well, by using gold earned exclusively in-game I've already managed to buy everything currently available (with the exception of some chest-only cards): both adventures, the 10k character pack, all five of the extra characters and a bunch of treasure chests for good measure, and it wasn't particularly onerous, nor did it even take that long in the grand scheme of things. If your primary goal is money, though, it's best not to try to earn that much using a full party though, as those runs — while epic adventures in themselves — take far too long to be efficient.
That said, given how easy it is to cheese the system and race characters through the easiest basic scenarios over and over again in mere minutes at most (and multiple people seem to have simultaneously figured out the same optimal path), the developers might want to look into implementing temporary diminishing reward returns for successful scenarios in order to force, er, encourage players to keep running different scenarios, rather than spam the same quick one endlessly in order to earn gold at almost exploitative levels. Not to reduce the overall rewards, but just to incentivise people to go about earning them in a way more in line with the nature of the game and gameworld.
Another option could be to balance the rewards around party size as well as difficulty, or even to highlight one particular legendary scenario at a time and reward an extra 50 gold or so for completing it, and have that bonus scenario change every time you finish it (whether you win or lose), so you're always chasing a different adventure for your reward.
Providing an in-game way to earn the things that can also be bought with real money is very welcome and more than generous, but as things stand the monetisation scheme is actually a little too unbalanced in favour of the consumer (as weird as that sounds), and the more people figure that out, the less they're likely to spend their own real money, as always happens.
As a side note, while I was on my way to earning the 10k pack I managed to complete every legendary scenario, and was really impressed by how close they often come, and how much things can look like they're simply not going to work out right before they actually do. Now I need to gear up the rest of my stable so I can take on a full-party legendary run.