The real monetary value of a free spins package.
Casinos rarely state the bet size behind free spins — and that's what determines the gift's real value. 50 spins at $0.10 and 50 spins at $0.50 differ fivefold, even though both get marketed as "50 free spins".
The formula is simple: number of spins × bet per spin × slot RTP — that's the package's expected payout in money. Compare bonuses across casinos by this number, not by the spin count.