Mike Perry:
Invest in the Tor network.
Based purely on extrapolating from the Noisebridge relays, we could add ~300 relays, and double the network capacity for $3M/yr, or about $1 per user per year (based on the user counts from: https://metrics.torproject.org/users.html).
Note that this value should be treated as a minimum estimate. We actually want to ensure diversity as we grow the network, which may make this number higher. I am working on better estimates using replies from: https://lists.torproject.org/pipermail/tor-relays/2014-September/005335.html
Automated donation/funding distribution mechanisms such as https://www.oniontip.com/ are especially interesting ways to do this (and can even automatically enforce our diversity goals) but more traditional partnerships are also possible.
Result: 100% capacity increase for each O($3M/yr), or ~$1 per new user per year.
Naif's point about there being 100Mbit residential uplinks out there suggests that there may be a hybrid approach here.
If this vendor could detect super-high-speed client uplinks, they could ask only these users if they wanted to be non-exit relays. But this is complicated, as it also requires understanding if the user's ISP will get upset at the traffic consumption or the fact that a listening TCP service is running. For example, I know Comcast calls their residential service "unlimited", but yells at you if you transfer more than 250GB in a month, or if they discover any listening TCP ports on your IP address.
Even if we could figure these problems out by looking up ISP policy based on client IP address, I think we still need to fund exit relays. I don't think we can just enlist random home users connections to be exits without giving them a wall of text explaining how to deal with issues that may arise.
So this may be something to consider to reduce network expenditure, but it won't completely eliminate it.