The staff got their numbers wrong.
The challenge name is "Unlimited Community Pool".
If base is $500 and each valid entry adds $15 to the pool after 100 valid entries the poll will be
100 x $15 : $1500 + $500(the base) = $2000 (instead of what's stated in the post of $1500)
after 250 it will be it will be : 250 x $15 : $3750 + $500(the base) = $4250 instead of what's stated in the post $3000)