Apparently the value changed slightly in one of the recent updates.
I assume this is done to promote people at least updating their mines a little bit??
The first 3 levels now have different values (since map revision 13 and up):
+-------+-----------+-----------+
| level | old value | new value |
+-------+-----------+-----------+
| 1 | 3,3 | 0,99 |
| 2 | 5,2 | 3,12 |
| 3 | 7,7 | 6,16 |
| 4 | 10,8 | 10,8 |
| 5 | 14,5 | 14,5 |
| 6 | 18,8 | 18,8 |
| 7 | 23,7 | 23,7 |
| 8 | 29,2 | 29,2 |
| 9 | 35,3 | 35,3 |
| 10 | 42 | 42 |
| 11 | 49,3 | 49,3 |
| 12 | 57,2 | 57,2 |
| 13 | 65,7 | 65,7 |
| 14 | 74,8 | 74,8 |
| 15 | 84,5 | 84,5 |
| 16 | 94,8 | 94,8 |
| 17 | 105,7 | 105,7 |
| 18 | 117,2 | 117,2 |
| 19 | 129,3 | 129,3 |
| 20 | 142 | 142 |
+-------+-----------+-----------+
Note that these levels are the base level for the mine, so multipliers should be applied in the order:
a) sum of techs and advancements
b) supercharged mine (x20 multiplier)
c) mine doubler (Rock Candy reward from Hardened Far Land)
d) ad watch bonus (+20% or +30% depending on Couch Potato reward from Hard Siege of Feldmere)