With only 10$ for starter set and no investment, I think you will be nowhere near D3 league. You would need to invest some of earned DEC into useful monsters, then I see you closer to D3.

In the first month, that is 2 seasons, I see G1 possible while G2 most probable. And I expect you to touch D3 after the second month because I believe that your experience will be enough for making stronger teams by that time.

I wonder how much of this will be fulfilled when coming back to this after two months.