Level 20 is less than 600 gralats an hour, it's barely better than level 0. It's not a good job until you are around level 80.

Here's what I mean about all the money you could have earned with other jobs while you spent all the time mining to get up to level 100

If the other job pays 1000 gralats an hour by the time you reach level 100 mining you have made more money mining
If the other job pays 1500 gralats an hour you have to mine another 49 hours (another 120,000 rocks) after reaching level 100 before the money made is equal
If the other job pays 2000 gralats an hour you have to mine another 161 hours (another 480,000 rocks) after reaching level 100 before the money made is equal
If the other job pays 2500 gralats an hour you have to mine another 290 hours (another 700,000 rocks) after reaching level 100 before the money made is equal
If the other job pays 3000 gralats an hour you have to mine another 436 hours (another 1.1Million rocks) after reaching level 100 before the money made is equal

So if you are in it for the money, mining takes a long time to catch up to other jobs even if you reach the max speed at level 100.
Mining starts out slow and until you reach level 80 is falling farther and farther behind what you could make in other jobs.