So, say 888 casino gives you a $20 bonus just for signing up. But they require you wager that 20x before cashing it out. That means you'll have to do $400 of wagering. So, if you play $.25 Jacks or Better and bet 5 coin you'll be betting $1.25 a hand. So, you'll play 320 hands then cashout. Might lose it all, but no matter. You didn't risk your money. Jacks or better RTP is 99.54%. So, on average a player doing this would cashout $18.16 if he plays optimal strategy on Jacks or Better. There is variance of course. You might lose the $20 bonus or you might hit a royal flush and win $1,000.
The ideal bonus is one that requires only 1x of wagering before cashing out.