^Good point, I forgot that Battle of the Ronin technically has limited prizes. I don't know how significant this is in practice. Looking at the leader board, only 5 people have completed level 14 of "Ronin" so far ($120 for 3000 points), yet there are 50 of those cash prizes up for grabs. So it doesn't seem there are enough high volume grinders on Lock (yet) to exceed the prize quotas. I'm not sure if this is true of the first few levels as well (it could be that all those $1.50 and $3 bonuses are gone by now), but so far I've been able to make it through the first 4 levels without any problems (and I'm by no means a "hi-roller").
If you started playing at the beginning of the month, you probably wouldn't have much trouble being among the first however many to hit the targets, even playing only casually. Merge has a million different skins, and I reckon Lock regs constitute only a fraction of its total player pool.
