This is a very well known pic. Again its from imageshack, can't give credit cause don't know the author.
It explains itself pretty well, i might just add just two things.
Upgrade success for
+11 is 5%
+12 is 4%
+13 is 3%
+14 is 2%
+15 is 1%
The other values remain unknown for me.
Also wanna add this table, here are the points you get each upgrade
Upgrade | Points |
+1 | 5 |
+2 | 5 |
+3 | 5 |
+4 | 5 |
+5 | 8 |
+6 | 8 |
+7 | 10 |
+8 | 10 |
+9 | 12 |
+10 | 12 |
+11 | 15 |
+12 | 15 |
+13 | 18 |
+14 | 20 |
+15 | 25 |