Not a rule - either way.
As in not required to give pricing, and not prohibited to give pricing.
Most of the time it's sort of obvious from context, tone, and tenor of the post whether some place is pricey or "budget-friendly." If one wants specifics, best just to look at the restaurant menu, or Zagat or some other source.
I, for one, do not post prices for a variety of reasons, one of which being that it may make people uncomfortable (myself included). If, for example, you went to a nice restaurant and you ended splurging a bit and spent something like $10000 for two people, you can either come off as (1) bragging and showing off or (2) duped for spending so much. Why inject that aspect of a review in the discussion -- i.e., whether the price is you paid for the food was worth it.
Now, of course, that subject is not verboten per se, but just that it doesn't need to be in every single post unless the OP wishes it to be -- e.g. "The $15 burger at ERB is not worth the price of admission."
But if someone just wants to talk about the food, the experience, and the enjoyment of dining out, who are we to complain?