Typically prices increase at many restaurants in January. Check out the menus and you will see the difference.
I find that many Expats possibly do not understand how much they are actually paying for their food. Pesos can be confusing and later if you do the conversion your meal may not be that inexpensive for the area.
Maybe it is less expensive that your native country, but in general when a meal costs $20 USD for lunch that is not really that cheap.
Ajijic seems to be more expensive than other areas like Chapala, Jocotepec or San Juan Cosala in general.