At very late stages, a company valuation is usually a multiple of EBITDA (earnings before interest, taxes, depreciation and amortization). The valuation (which is different than market cap, but we'll conflate the two for sake of explanation) in very late stage, non-growing companies, during times of slow economic growth is around 5-7x. If a company is growing quickly at the later stages (read: exponential growth curve) its P/E ratio (price/earnings ratio) could be, we'll say, 100 on the public markets (100x). That's really high, and would rarely happen unless the economy has really low interest rates (meaning less good places to put your cash, and you need to invest it somewhere anyway).
So, in other words, we give the company the benefit of the doubt and say they'll be worth 30x EBITDA. That means they'd need to bring in about 20m in one year, which is about 400m in total donations. High for sure (the buyers must really believe in them), but not completely unreasonable. If, on the off chance, they start bringing in 1B in donations, and their take is 50m, the investors will earn a great multiple on their investment. That's probably what they're hoping for.