Each 1k rocks give you around 3000-5000 gralats ( I actually tried lots of time to get the exact number).
You can get exactly the amount of your benefit based on the certain number of rocks you decided, but you can't caculate the amount of benefit between level 44-99 in average, and it also depends on how long you need to pick up 1 rock per level.

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There are some percents of getting nothing too lol