My Lands: Black Gem Hunting

Usually there are discounts the larger the purchase that is made in games. In looking at the cost of different tariff packages of BG, you actually get less per $ spent once you go over the 1800 BG mark. It's not a big difference, but I'm kind of curious, is there is a reason for this?

For example:
100/2.19 = 45.66 BG per $
650/10.79 = 60.24
1800/19.79 = 90.955
3000/32.99 = 90.936
4200/46.19 = 90.928
10000/109.99 = 90.917
100000/1099.99 = 90.909
...

We don't provide such information.
You may choose a tariff, which is the most convenient for you.