SecondMarket is a different animal -- they structure liquidity tenders for late-stage private companies and collect portal fees per seller, and, in the past, ordinary secondaries transaction vigs. They also run a large, traditional financial services business that has historically paid for much of their involvement in the startup world.
Funders Club is also a somewhat different animal -- they offer investors access to venture funds where risk is pooled, and also do single-purpose funds that resemble your product.
You are exclusively offering investors access to one investment and one set of risk. Let's say you can get it down to $4k/yr:
* Accountant to prepare and send out K1s to 50 shareholders $1000
* Delaware agent & franchise tax $400
* Legal, regulatory filings, 4 hrs $1000
* Your or your lawyer's time to give legally binding answers to questions from 50 shareholders $?
* Valuation analyst hired by you or firm, 5 hrs $600
* Broker dealer / misc / insurance $1000
These numbers sound pretty low to me but let's say you have economies of scale.
Making money solely when investors make money still seems like a reach. If you make 100 investments per year and have a decent team you will be burning a lot of cash -- the only way to recoup that cash will be to charge a transaction fee or keep raising money until carried interest pays out (which it often doesn't). Either way, it's something you should address in your disclosures.
It's one thing not to have a fleshed-out revenue model if you are a consumer site: nothing will happen if you die except for some upset users, but you can't just suddenly unwind a group of 400 investment funds when the underlying capital is tied up in illiquid private stock.