If you get a 100% bonus on $100 (so $200 total) with a 20x wagering requirement, you need to wager $2000 before withdrawal. Playing blackjack with optimal strategy has about 0.5% house edge, so mathematically you’d expect to lose 0.5% of $2000 = $10 during the wagering process. So your expected value is +$90 ($100 bonus – $10 expected loss).

HOWEVER, this is why most casinos now exclude blackjack from bonus play or count it at reduced rates (like 10%). If you play slots with a 5% house edge instead, your expected loss becomes 5% of $2000 = $100, which completely negates the bonus value.

This is why casinos have “smartened up” over the years and reduced the profitability of bonuses. The golden era of bonus hunting is long gone.