Ódýr

I'm not aware of many languages that resort to prefixation as a means to express the opposite of expensive, id., cheap. English has inexpensive, but cheap gets much more mileage. Esperanto makes extensive use of prefixed adjectives: sana (healthy, from Latin sanus, a, um), malsana ("badly healthy", "malhealthy", that is, sick, ill), bona (good, from Latin bonus, a, um), malbona ("malgood", that is, bad), etc. The one European language, though, I've discovered that differs from others in this regard is Icelandic, where expensive/dear is dýr, and cheap is ódýr, "undear."