Hi Greg,
Dry goods pricing may not deviate appreciably from week to week. Produce prices will fluctuate contingent on weather and seasonality. Protein pricing, seafood in particuliar, can/will swing unpredictably.
For example, 2~3 weeks ago, price of Dungeness crab in local markets (San Francisco Bay Area) was at $12+ per lb. Today, same can be found for in the same tanks for around $6/lb. Granted this a retail perspective. From experience, restaurant buying prices will track a similar price curve (albeit with a wholesale discount).
Many restaurants do not have the resources to reprint their menu daily/weekly to track market food costs fluctuations. If an establishment prints set-in-stone menu prices for a definitive time period, then they may (WILL BE) accused of price gouging if/when the market price of the commodity gets halved (as in crab). Or, they will bleed red selling food at a loss when prices escalate.
All said, I rarely order anything listed as market price.
