One of the things naysayers say about mars colonization is resupply kills it (like the recent MIT study.) So an annuity that pays forever seems like a natural answer to that complaint. I see annuity rates now at 7%, so 5% seems a reasonable return to me. The only show stopper is funding that annuity. The $20b spent on SLS/Orion would have more than done it, but that's been pissed away already.
I strongly believe we could send a dozen colonist to mars every 26 month launch window for about one billion dollars per mission. A 5% annuity gives us that for under $10b. If I'm wrong, we still go, but at a slower rate unless we do get sufficient funding.
This is more than you can expect from a kickstarter campaign. It's a lot less than the idiotic numbers others throw around (they're real purpose being simply to discourage.) Interestingly, it's not far off from the six billion Mars One is looking for (people doubt they can pull that off either.)
But is it too much to give humanity a whole new world? No, and private investors will one day realize this.