I can’t seem to find a simple, but idiomatic way of saying this. It’s kinda hard, so if you don’t understand what I meant by the title, just look at the example below.
I get to spend 100$ every month from dividend earnings. (I earn
dividends 4 times a year, but the total earnings yearly divided by the
number of months which is 12, is 100$).
The word that will be useful is “equivalent”, but whether you need it depends on exactly how much of this information is important, and how much you can let the reader work out for themselves. For example you could just say
I get $1200 dividend per year.
And let the reader do the maths. Similarly, you could say
I get $300 per quarter.
If you want to be more explicit then it is possible to say something like
I get $300 per quarter, equivalent to $100 per month.
The fact that it is equivalent could be implicit:
I get a dividend of $300 per quarter so I get to spend $100 every month.
If you only want to mention the $100 then
I get the equivalent of $100 dividend per month.
The idea of an “equivalent” rate is fairly common in finance, where (for example) interest rates are often quoted as an “Annual equivalent rate” (AER) for easier comparison. The process of converting a financial rate to the yearly equivalent is “annualizing”, but I don’t think there is an equivalent term for calculating a monthly amount.