Not necessarily - on lower levels cup can earn you a lot. I am level 2 and I've put a lot of effort (and money) in getting my team to 5* and especially prepared them for the first cup game against a level 4 team, which I managed to win 1:0, 2:2. From these and the next two cup games I will earn around 8M which is a massive amount of money for level 2... (luckily my 2nd squad won my league games too )