retkost

1. rarity; rare thing
   Ovakve ploče su danas prava retkost. — Records like these are a real rarity today.
   Nije retkost da voz kasni pola sata. — It's not unusual for the train to be half an hour late.
2. rareness; scarcity
   Cena zavisi od retkosti marke. — The price depends on how rare the stamp is.

The phrase "nije retkost da…" means "it is not uncommon that…".