If you go even higher, you can get even more expected value, but in sports betting, you only have a finite number of bonus bets. If you bet a $50 bonus bet on each of these lines -250, +250 and +500 - you're going to get more expected value the higher you go (again, we're just multiplying the probability times the win amount to calculate the EV). There's a lot going on in this chart below, but it's just like the coin toss example. That would result in $1,000 over 1,000 flips. You're still winning either $2 or $0 on every coin toss, but the EV is $1 because over 1,000 flips, you'd expect to win 500 and lose 500. 5 (since there's a 50% chance it lands on heads).
If I flipped a coin and got paid $2 for every heads and $0 for tails, the EV of every flip is $1 - $2 x. Expected value is just probability multiplied by profit.
