What's a common idiom for "price of admission"?

Are you talking about de rigueur?

required by etiquette or usage or fashion

In certain contexts, this might fit the "basic requirement" part.

You could be thinking of the borrowed French phrase prix fixe, which in English refers primarily to a certain kind of meal/meal service or the price charged for it, but it also means literally, as it does in France, “fixed price” or “the price charged” of/for anything. (from Merriam-Webster)

Another possibility is the use of the term “cover charge or just “the cover” (and sometimes the French “couvert”) to mean “an entrance fee sometimes charged at bars, nightclubs, or restaurants.”
(from Wikipedia, Google Books, and Wiktionary, respectively)