>there is a lot of interesting exploration and science that NASA could continue to do that no profit incentive exists for (deep space probes, space telescopes, etc.),
A lot of the historical "voyages of discovery" were financed by rich people looking for glory, newspapers looking for exclusive stories, and companies looking for good PR.
I would first note that more 'voyages of discovery' were prompted by the desire to establish new or improved trade or resource exploration routes, and we've pretty-much established that there's no one to trade with and nothing to exploit within reach.
And I would like to see the cost comparisons, but I'd bet that just as pan-oceanic trips were an order of magnitude more expensive than explorations before, space journeys (especially manned), would require an exceptional economic incentive to be at all viable.
A lot of the historical "voyages of discovery" were financed by rich people looking for glory, newspapers looking for exclusive stories, and companies looking for good PR.