Ten times that wouldn't be worth it, if it distracts from a single other important task. Assume $2400 / hour, six hours per day of effort on it, five days per week, 52 weeks per year. $3.7 million. It's a lot to a charity, it's not a lot versus his net worth and what he can do with his time at A16Z, against the distraction of writing emails for that effort every 5 minutes (which is why he won't continue with this long-term; he'll make a splash with it, get the headlines, do a good deed with the donated funds, and be on to the next thing, which isn't writing emails for $5/$20/$50).
Andreessen Horowitz put $25m into the B series for the company running this service, which puts a different complexion on the ROI he's hoping to get from answering a handful of emails in his free time...
tbh I wouldn't be that surprised if some of the companies willing to spend $20 to exchange emails with Ben Horowitz are actually worth his time replying to, especially if he has a PA he trusts to decide which emails are worth him having a look at.