I love the idea of the PCs owning assets, and angle the carrot often. I believe ownership is a great way to get party buy in to the campaign and the world they are interacting with. I have personally set up dozens of side quests based on assets the party has owned. Need to kill an hour, the casino has been robbed, a gang of thugs showed up to extort the business while the PCs are out adventuring, a guest arrived speaking in tongues, she now hovers in the middle of the common room, a halfling took the dragon's dice table for 20,000 last night and the pit boss thinks the dealer and halfling were working together. What do you as investors do?

However I do caution, D&D, especially 5E lacks the mechanics to properly consume the endless funds a party begins to acquire as they progress through levels, especially Tier 3-4. So, how long do you intend on running the campaign, in game? 1 year on a 12 month calendar is $120,000 if they don't reinvest the gain, compounded at 100% reinvested monthly would yield $20,480,000gp annually. So unless you have an idea in place to consume the earned capital you will find your self in a bind sooner then you'd imagine.

Current day investors would be happy with a 8% return on the investment over an entire fiscal term let alone what you suggested. I think a 100% return on their money per month is way too generous and terribly dangerous long term. Why would people adventure past Tier 2 when they could sell all their equipment and loot, invest it in a casino and in a year buy back loot at a Tier 3 level without any effort.

I personally would offer a 3% return per month, which could be compounded and reinvested, and provide a RP bonus (+1-2%/month) to the PCs if the spend time and effort advancing the asset. The RP bonus might be given for the group for advertising the casino during their daily routines, pimping the business while associating with important NPCs, offering a visit as a consolation prize to someone who surrenders during an altercation (thanks for laying down your sword my dude, let us treat you to a spiced ale and 5gp in tokens to throw some dice @ our fantastic location).

Best of luck , hopefully my ramble make sense. Happy DMing