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.