Is it worth it to purchase a coursera certificate in Machine Learning for grad school apps?
By and large, no. Buying the certificate is meaningless with regards to graduate admissions. You can list MOOCs that you have taken on your CV but do not expect them to carry any weight whatsoever. Admissions will only pay attention to accredited courses.

Note: I say this both as someone who evaluates applicants and as someone who is a lead educator on a MOOC.