The Frostbite is a one-handed sword weapon dropped by the Alpha Icewhal on Floor 4 at a very low drop rate, which is increased even more so due to the long spawning time. It is extremely desirable and sought by many. Frostbite used to be an easy sword to obtain because of dupers duping them and cost way less now, than its average price before.
It does not cost just 5 mil. Regardless of duping, an acceptable standard price of 30 mil still stands for min AUSP. Anything less than that is either a scam if you're selling, or if you're a buyer than the seller is being generous.