If you're wanting to present something to the DM, as he's already a very busy person, then I'd personally come up with some kind of high level abstraction.
For example your character could invest a certain amount of money for a variable monthly return. That variable return could have a few simple inputs.
Example: Player Owned Inn
You spend 10,000 gp on purchasing an inn. Agree with the DM a base level of return, say it makes you 500 gp a month.
Then add in some variation. If you are there for the whole month, buttering up the customers and jollying along the staff then you make your full amount. If you are off adventuring the DM could lower it.
Or the return could be based on a dice roll instead, such as 1d6 time 100 gp.
A chain of inns could introduce some adventuring plot hooks even. Eg. the manager of your inn on the other side of the realms is skimming off the profits, or not returning your cut to you at all. So you have to go sort him out. Turns out he's being blackmailed by a high level organisation or BBEG that you party then needs to deal with.
As a DM, a player that comes to me with potential plot hooks is a player that's more likely to get what they are asking for.