Never say never. Given that we now know (since 1985) how to use gravity assists to get to Mercury, a manned mission may not be as impossible as you say. After some searching I found that 590 kg of fuel were used for the Mercury MESSENGER trip. In comparison, the Mars Reconnisance Orbiter used 1149 kg, Mars Express used 370kg and Cassini used about 3000kg. So there doesn't appear to be any big difference from missions to other planets in terms of fuel use or expense, just in the travel time. If there was a good reason for going to Mercury, people could withstand a 6.5 year journey, given suitable provisions. Perhaps the reason would be to start an underground colony near the poles and water deposits. At the right latitude, and a couple meters underground, there would be constant room temperture. And there would be ample sunlight for solar power and agriculture. For a 6.5 year journey, protection from ionizing radiation would probably be the biggest obstacle to overcome.
