from what it looks like...not really a good investment. for those who think you need to net a loss on diamonds, i would disagree. the giant cave should net you diamonds (not a lot, but surely net some) and lots of gold. why? basic investment strategy, you give me something, and you wait a while. your investment will increase.
since a free 10hour cave nets you a few diamonds, i would think giant cave should gross 120% so your net is 20% that would seem fair. plus lots of gold for paying into it.