I only buy when is doble lf4 event and ALWAYS do the same, this is my last event day:
first step: buy 40 keys and collect botty only in x-5 maps, result?? 2 lf4, 3 shields elite, cpu, ammo,...
second step: buy 40 keys again and collect in the same map, result? 3 lf4, 1 velocity drive, ammo, rockets,...
third step: the same as before, result? 0 lf4, 4 velocity drives, 3 shields of 5.000 uri, ammo, rockets,....
Fourth and last step, same as before, result? 6 lf4, 2 velocity drives, 0 shields, ammo rockets,...
There is no trick, but the next time I will try to do the same and you already comment on my results.
In total it took 11 lf4 with an investment of 240,000 uris (is not so bad)