I think it is one of those diseases that flairs up. However, I have sent swabs for analysis on all my Princesses who died this summer and none were positive for Mycoplasma.I daresay price would vary widely from market to market. I may ask for an MG swab of Janet's bacteria when she has her checkup next weekend.
I saw @Perris commented that MG is probably in most chicken flocks already. I guess healthy birds aren't bothered by it.