1) Reasonable cost is what taxpayers/voters are willing to give. If you want a $100bn NASA budget, you are basically asking every American for $200/y. If you made that optional, I'd argue that a
lot (most) Americans would
not be willing to pay.
2) I see no probable route for fusion reactors to become a competitive source of terrestrial electricity for at least the next 50 years and possibly never; without that, Helium-3 is mostly worthless (even if your fusion bet works out, you rely on an approach winning that actually needs He3 instead of breeding its own Tritium). For everything else, I don't see extraterrestrial mining being able to compete with current prices, and any significant influx would have it crash/undermine its own market (e.g. we only extract hundreds of tons of palladium globally, per year; doubling the supply would have a major effect on price).
3) I'd argue that current Moon/Mars project are mostly ineffective showmanship/PR. If you actually wanted somewhat self-sustaining settlements/industry within the century, costs would easily eclipse our current defense budget, and without demonstrating the ability to build that on earth first the whole thing would not be credible anyway.
Our current approach to manufacturing (post industrialization) is totally incompatible with self-sustaining colonies, too. There is nothing we could realistically achieve on moon or mars even in a century that is anywhere close to self-sustaining, without basically reinventing how we build things.
So from a risk mitigation point of view the whole endeavour is useless, too (this might change within a century-- synthetic biology specifically would be very promising here).