This math really just goes to prove that your offer is ridiculously bad for her. I mean you'd essentially be paying just above her opportunity cost from not investing the money in other potential investments, so she'd not actually be making much more than a safe investment.
And that's before accounting for the massive risk of failure that her other investment options don't have.
3.75% of gross profits per mil (equivalent to the original 5% of benefits per mil) approximates a 12% per year investment return rate. 12% is pretty good in terms of real investments, though probably a little weak for a magically gifted investment. Regardless, that's her opening offer, so she at least considers that reasonable.
If she's looking at truly long-term benefits, though, her return continues to increase over time, as long as we maintain a steady growth rate. The problem is that there are limits that we'll run into that I'm not sure how we'd expand past. (Not that I don't think that we
could, I just don't know how it would happen.)
If I cap average income at $48k per year (which happens about 12 years out at the growth rate I have set up), total repayment over 20 years at the 3% per mil rate would be $7 mil, or an overall average growth rate of 10% per year.
If we added extra territory and capped our population at 1200 instead of 1000, it would be $8 mil over 20 years, for 11% per year, using the 3% payment rate. 3.75% is back at the 12% per year rate, hitting $10 mil after 20 years.
If we did something crazy and absorbed Nagoya or something, putting our total population at around 2000, overall rate of return only goes up to about 13% per year.
Basically, growth rate per year slows down very quickly barring truly exceptional circumstances. Getting above 12% will be hard, and even increasing our payout by 25% (from 3% to 3.75%) only increases the growth rate from 10% to 12% (so a 20% gain).
If she's aiming for a 10%-12% return rate over a very long time (20 years or so), we can work in the 3% to 3.75% range. $2.7 million would incur an 8.1% repayment rate even at 3%, though, which is getting noticeable.
In general, we spend about 80% of our income on our members. 3% on $2.7 mil would increase that to about 88%, while 3.75% would increase it to 90% (10% payment value). With 600 girls, 12% of $300k is the equivalent of 20% of $180k, which would be 360 girls. If we can expect this to get us the needed funds for more than 240 girls (considering 360 is a bit above my outer stretch for what we could afford on our own), it's worth it.
Anyway, if she's OK with the 12% return rate (3.75% per mil) as an opening offer, we should be able to counter with a 10% return rate (3% per mil). After that, it's how much we want to draw on. I'd say $2.7 mil (8% of our profits) is probably about the max we want to allow for.
Probably still want to cap the duration at between 10 and 20 years, though. She'd be making a pretty tidy profit (at least $7 mil per mil, over 20 years), but it's not like we didn't make a pretty solid concession to Hiko for a similarly critical bit of assistance.
I hope you're all happy, I'm now spending time trying to calculate how much return on investment I'd be able to get if I knew in advance Tokyo would be obliterated. Taking into account borrowing money to buy on margin to short sell. And oh god the ROI.
I was gonna say, it's kinda hard to sell those shorted stocks when the stock exchange is underwater, but I guess there's a chance she might manage at least some of it before it all became worthless.
. (We can almost compete...assuming we're completely riskless 
.
