A negative number is the stake required to win 100 units. A positive number is the profit on a 100-unit stake. -150 and +150 are not mirror images: the first implies 60% and the second 40%.
Decimal odds express total return per unit staked, including the stake. 2.50 returns 2.50 for every 1 risked, implying 40%. Implied probability is simply 1 divided by the decimal price.
Fractional odds express profit relative to stake: 3/2 pays 3 profit for 2 risked. Convert to decimal by dividing and adding one, then invert for probability.