Yes, PREMIUM is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, E 1, M 3, I 1, U 1, M 3.

Is it a valid word?

PREMIUM is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

payment for insurance
the amount that something in scarce supply is valued above its nominal value
a fee charged for exchanging currencies
a prize, bonus, or award given as an inducement to purchase products, enter competitions initiated by business interests, etc.
payment or reward (especially from a government) for acts such as catching criminals or killing predatory animals or enlisting in the military

Also: insurance premium, exchange premium.