It would be interesting to know what Mashgichim get paid in each Hashgacha and whether the Hashgacha or the restaurant sets that rate.
I know in FL, the official starting rate is $18/hr. However, if you find your own mashgiach and have them certified by the hashgacha, you can set whatever rate you want. The food establishment always has the option to pay more, which many do in hopes of getting more non-hasgacha production from the mashgiach.