Real estate values in San Miguel de Alllende

The real estate market in Mexico is not regulated, as it is in the USA, and you will be unlikely to find any of the 'statistics' that you seek. Just pick out the house you really want and make an offer; it is the only way you will know 'what flies'.
Since most homes are owned outright, without mortgages, many owners are retirees, and maintenance costs are very low, there is little pressure on sellers to move quickly.
Welcome to the forum and to Mexico.

For all the reasons RVGRINGO mentioned, people selling have a lot less motivation to cut prices. There are exceptions where a medical or family problem but the people that I talk to would just as soon hold out as costs very little to do nothing.
That being said, I would get a copy of the Attencion as the largest amount of homes and you will see which are being reduced. This will also give an idea of how much and in which areas.
Then, you can try to low ball to see what happens. My experience is that this works better with expats as locals can get offended.

That last comment is very true. If the seller is Mexican, you can bet that a 'low ball' offer will not only offend, but that the entire neighborhood will soon learn about it. Expats may 'dicker' but Mexicans usually decide on a price and stick to it.