Numbers Beyond Comprehension

(An ongoing scheme remains face up until it's abandoned.)
At the beginning of your endstep and when
this scheme is set in motion, create
three 1/1 colourless Eldrazi Scion creature tokens.
During your opponents turn, they may pay a
combined total of {x} mana, where {x} is equal
to half of the number of creatures you control,
rounded up.
If they do, abandon this scheme.