Starting with a 240% soul bonus on your stat page, calculate buying a 50%, 40%, 30%, 20%, and 10% bonus.

It's not 50% 40% 30% 20% 10% (this would be 150%).

It is x1.5x1.4x1.3x1.2x1.1, or x360.36%.

Since the bonus doesn't include the base 100%, an enemy that has 100 base souls will yield 340 souls from a 240% bonus, and 1225 souls from a 1125% bonus after buying the upgrades.

340x1.5x1.4x1.3x1.2x1.1 = 1225.
340x3.6036 = 1225.

The following is what people tell me is the easy and intuitive way to calculate what your stat page will show. You take the starting value, multiply it by 1 plus the bonus (so 50% becomes x1.5), then add the bonus (another 50%). Then take the result and do the same thing with the second bonus (x1.4 40%), and so on down the line. Don't misplace your parentheses or you will get wildly different results.

((((240%x1.5 50%)x1.4 40%)x1.3 30%)x1.2 20%)x1.1 10% = 1125%

For completion I'll include the way I prefer. You take your starting bonus and add 100% (so 240% becomes 340%), multiply it by 1 plus the bonus (again, 50% becomes x1.5), do that for each bonus, and then remember to subtract 100% from the result.

((240% 100%)x1.5x1.4x1.3x1.2x1.1) - 100% = 1125%