I've noticed that there's a "price" tag and a "prices" tag. Is there any general rule about singular and plural tags?

Generally, the singular form is preferred but sometimes the plural form is used based on the wiki/context of which that tag is used.

As there is no wiki for price and prices at that moment, it should be used sparingly until such a time a definition is written for them. See also, price_tag & price_sheet.