Err, actually the silver "penny" (change name for locale) was the single most common coin during the Middle Ages (and beyond).
In England from the time of the Anglo-Saxons through until sometime in the 14th century, I believe the silver penny was the only coin minted (numismatists, help me out on specifics here!). That coin could be cut in two (ha'penny) and quarters (farthing); the shilling, mark, and pound were accounting concepts, but with no actual coins. France was in a similar situation; I become less sure when I start talking about the Italian & Iberian peninsulas, but I'm pretty sure the silver penny remained the standard.
Of late, just to put my mind at rest, I've instituted the following change in my games: keep the costs the same, but anything labelled as gp becomes sp, anything labelled as sp becomes cp and anything labelled as cp becomes "bits" (a very small coin of low worth). Gold coins are worth 20 silver coins and are only used for major transactions.
I try not to get too into economics.
...actually, given my players, we keep very little track of day-to-day coin-counts. We run low-level games and only worry about having "enough" to get food and lodging. Personal gaming tastes only, of course.