It is possible to do that here in Zihuatanejo. I know three people that are/have done it successfully.
One couple mistakenly thought they could buy their permanent residency here in the country. Nope. They paid the fines for overstaying their 180 days last June, 4,500 pesos each. She went in again in January to get another extension. It was possible, but the cost again was 4,500 pesos including fines. She did not renew her permit, I don't know what they plan to do.
A friend's 180 day permit expired yesterday. Last week she went to Immigration and asked about what to do, nicely. They required her to provide 6 months worth of bank statements, and told her to return today, the day after the permit expired, and told her that since she was getting renewed due to "Humanitarian" reasons, the cost was 2,100 pesos. She was shown a list of fees for various reasons. She just returned, it worked.
So, what does it all mean? At least here you can renew your 180 day FMM for a price, which is flexible.
A friends son lost his FMM (and passport) and had to pay the fine in Mexico City before flying out. The cost was the standard 600 pesos.
