Decimal odds include the stake in the potential return. At 2.50, a winning stake of 100 returns 250, of which 150 is profit before any fees or taxes. The same price is +150 in American format or 3/2 in fractional format. A losing settled stake has a different outcome: this is not expected profit.
The reciprocal of 2.50 is 40%, an implied probability rather than an independent forecast. For a full market, add the reciprocals of all mutually exclusive outcomes from the same bookmaker and timestamp. A total above 100% is the overround. Dividing each reciprocal by the total removes it proportionally, but does not reveal the true probabilities.