Question from a start-up ratings site. how does yelp assign price ratings?

e.g. restaurants are rated as $/$$ etc. Menupages.com does the same.

We are building a specific kind of ratings site and want to add this feature. Just wondering if this is a manual process or a formula based on the avg cost of a meal/room. Any guidance is much appreciated.

It is likely to be some ranking formula, based upon an "average" purchase. For example, a hotel will be for one double room, while a restaurant will be for two courses, etc.

If you have a big data-set it is trivial to work out the ranking boundaries, but otherwise you could just assign reasonable ranges and adjust later if necessary.

Alternatively, crown-source it by asking your visitors to vote.

They clearly use their own rating/sorting mechanism. There is a lot of info here

There is a whole science behind reputation systems. Understanding and setting rules about freshness, validity, and attack protection are important if the system is going to be more than a simple graphic.

There is a good oreilly book on reputation systems by farmer and glass worth reviewing.