Cost of upgrade in Silver Coins:

1lvl  2% bonus

1->2  3 Silver Coins  3%

2->3  5 Silver Coins  4%

3->4  10 Silver Coins  5%

4->5  20 Silver Coins  6%

5->6  40 Silver Coins  7%

6->7  60 Silver Coins  8%

7->8  80 Silver Coins  9%

8->9  150 Silver Coins  10%

9->10  ??  Silver Coins  20%

I need your help.