It shouldn't be objective amounts because that would effect basically nothing in terms of difficulty. I like the idea, but due to how "speed crusaders" work, even decupling (x10) area requirements would just make an entire run last...maybe an hour longer? Two hours? Players with a good number of speed crusaders will be increasing spawn speed a ton, consistently getting 4-8 rewards per enemy, picking items up faster, etc, and boosting an area's requirements just helps the "Idols Over Time" talent to accumulate more. Rather, I would suggest boosting the way enemy health and damage scales. Sliding the difficulty up means that people with enough idols/gold find to max crusaders pretty early would end up hitting that no-more-insta-kill wall much, MUCH closer to the point where they hit the level cap, and having an extra-stacking idol booster as a reward would probably net more idols than progressing X areas without gaining any levels.

However, the rewards should not be on a 1-to-1 scale. I'm not doing the math out right now, but since the game's areas scale in difficulty pretty quickly, buffing enemy health and damage by x10 wouldn't really push advanced players back very far. I'd estimate someone that regularly reaches 1100 would maybe get pushed back to...1050? Probably not even THAT much if they are insta-killing at 1100 normally, and sacrificing 50 levels (which, overall, amounts to very few idols) to get a x10 multiplier would just be completely absurd. Rather, something along the lines of, like, 20% of the amount you buff the difficulty by, so boosting the enemy health and damage by x2 would only get you a paltry 0.2% more idols, while boosting by x100 (which would ACTUALLY make a dent in progression) would boost idols by 20%.

The idols lost by x100 difficulty would probably amount to breaking even with the idols received by the 20% boost, so I'll let someone else crunch the numbers to figure out an actually-balanced ratio.