Just $5 and i'm 27-30 on chart (i wasted $20 on peronamin before event, but i was bad balanced). The idea with cash is buying pudding and trolling some kind of cards (Moe in my case). In the other hand, event gachas are a waste of money.
with that boost + coliseum gacha i got the 200 killstreak in less than 2 days (well, someone got that before me).
FYI: I got a similar* tactic during elite guard event (without cash that time), taking some low level contacts instead of my own targets (pos. 359 in support ranking), using event rolls to take resources (just 3 stamina)