Rupees, as the currency of Hyrule, are ubiquitous to the Zelda games. They come in a variety of colors, with each color signifying a value; particularly large rupees are worth much more than their normal-sized counterparts. They are a universal currency in the land of Hyrule, being accepted by the kingdom of Hyrule itself, as well as far-off lands such as Holodrum and Labrynna, or even farther places - the plane of Terminia, or the dream-world of Koholint Island, also accept the rupee as payment for goods and services.
While many in these places would also accept treasure or valuable gems as payment, it should be noted that Rupees are not actually gemstones in the proper sense; they are more akin to glass than anything else, though minting a rupee is difficult and provides some of its worth in and of itself. Rupees also have some magical properties, which makes them useful to creatures not typically thought to have a use for material wealth, such as faeries. While they cannot be used in place of ritual components, they can be made into focuses for rituals, and often are by unimaginative or money-minded wizards seeking to anchor magically-closed doors onto something they have at-hand. Rupees typically weigh only a fiftieth of a pound, and are small enough that large amounts can be carried with little trouble in a sack or wallet.
However, much like any currency, their value waxes and wanes with the ages - a rupee of a given color in one era is not worth the same as it would be in another time. As such, it is difficult to pin down exactly how much a rupee is worth, and turn them into a currency that can compete with the standard gold piece in terms of usefulness, both economic and personal. Below is one such attempt, with the value of the least expensive kind of Rupee, the Green Rupee, pegged at that of a single gold piece (the most common kind of currency for adventurers to deal in). The below attempt has been made with the value of magical items in mind, so that the Rupee may be a valid currency to deal in from levels 1 through 30.
For the everyday economics of the average villager, Rupee Dust (an ounce of which is worth 1 copper piece) and Rupee Shards (one of which is worth 1 silver piece) suffice. For the economics of kingdoms, Big Rupees - at least twice the size of the average rupee - form the backbone of transactions.