The value of one block increases with the difficulty. This is a list of the block values until difficulty 20. The formula is rather simple: nSubsidy = ((0.2 * exp(nDifficulty / 250) / pow(nDifficulty, -0.25)) * 50), but working. The minimum block value is 10, the maximum block value is 50 (will be reached with appr. difficulty 110).
0.000000 = 0.000000
1.000000 = 10.040080
2.000000 = 11.987589
3.000000 = 13.319620
4.000000 = 14.370230
5.000000 = 15.255568
6.000000 = 16.031010
7.000000 = 16.727643
8.000000 = 17.364805
9.000000 = 17.955406
10.000000 = 18.508524
11.000000 = 19.030804
12.000000 = 19.527266
13.000000 = 20.001803
14.000000 = 20.457497
15.000000 = 20.896834
16.000000 = 21.321848
17.000000 = 21.734230
18.000000 = 22.135398
19.000000 = 22.526555
20.000000 = 22.908732