One million by level 40 is rather easily attainable if you know the order of enemies to kill for the majority of the levels up to that point. My usual is around 1mil by about 35-40, 2mil by about level 70 (depends on whether I get 3 big crystals worth at least 90k).
My personal best I got by getting all the large crystals for the first 20 levels (and I mean ALL), plus several large point items I got (1mil at 23). Then I got a money bag around level 25, and kept it up to level 65 or so. That was the first time I broke the 3mil point marker.
The next time I broke it was after I learned how to get the big crystals on the later levels, and just managed to get some really high valued ones.