I tried a few ways, it is impossible to save the money for the third level, because to get that much money you end up around waave 70-80 and you havent got enough towers to fend them off. That tower is not the way to go, sadly.
Do the math, it costs 23000 to get one 3rd level flower doing 909090 dmg. That 23000 could be spent on roughly 10 seed towers each doing 9090, so theirs no advantage.